Transcript for:
Foundational Concepts in General Chemistry

in this video we're going to focus on the topics that you've learned in the first semester of your College general chemistry course now this information can still help you if you're taking uh IB or AP Chemistry in high school it covers the first half of what you learned so it covers story geometry percent yield empirical formulas uh how to find the limited reactant how to convert from grams to moles to atoms or molecules it also covers dilution problems balancing equations um how to find the oxidation numbers how to identify if it's an oxidized or a reducing agent we're we're also going to talk about gas LW PV cor nrt we're going to go over um vapor pressure partial pressure grams law of diffusion mole fraction and the kinetic molecular theory we're also going to talk about solution story geometry and um we're going to go over marity mality Mass Le structures molecular geometry how to find the enthropy of the reaction um using Bond Association energy or heat AFF formation and even hess's law we're going to talk about all of that we're going to go over intermolecular forces boiling point vapor pressure like bowling point elevation freezing point depression osmotic pressure basically almost everything you've learned in the first semester of your Chemistry course so let's uh go ahead and begin number one how many protons electrons and neutrons are found in the ions shown below an ion is a particle that has an unequal number of protons and electrons in this particular ion we have a charge of plus three which means that there are three more protons than electrons an atom is electrically neutral in an atom you have an equal number of protons and electrons now looking at the numbers on the left side the smaller of the two numbers is the atomic number and the larger of the two numbers is the mass number now the atomic number is always equal to the number of protons so because the atomic number is 13 we therefore have 13 protons so we can eliminate answer Choice C and D because they don't have 13 protons to find the number of neutrons the number of neutrons is equal to uh the mass number minus the atomic number so in this case it's going to be uh 27 minus 13 so therefore we have H 14 neutrons so that really doesn't distinguish answer Choice a from answer Choice B so now the last thing we need to look at is the number of electrons here's an equation that can help you to find it the number of electrons is equal to the atomic number I'm going to write a n for that minus the charge so in this case the atomic number is 13 minus the charge of plus three so 13 - 3 um is equal to 10 and so therefore there are 10 electrons now makes sense if we were to add up the charges we have 13 protons and 10 electrons now the charge of a proton is positive and the charge of an electron is negative so pos3 + -10 that gives you a net charge of plus three so an ion with a positive charge has more protons than electrons in this case aluminum has three more protons than electron in its ionic form number two what is the correct name name for the compound n205 now for each of these problems feel free to pause the video and try these examples yourself and then unpause it to see if you have the right answer now before I give you the answer for this compound we need to talk about the naming rules for two different types of compounds and that is for molecular compounds and for ionic compounds n25 is a molecular compound it's composed of non-metals whenever you have a compound with a non-metal with another non-metal it's a molecular or calent compound electrons are being shared now let's say if we have aluminum chloride Al cl3 this would be an ionic compound because it's between a metal and a nonmetal now for n205 we would use prefixes like mono di Tri Tetra mono for One D for two tries for three Tetra is four Penta is five hexa is six hepta is seven OCTA is eight n is nine Deca is 10 so for n205 it's D nitrogen D for two pentoxide pens for five so the correct answer is D for this particular problem now for aluminum chloride al3 we won't say aluminum TR chloride that won't be correct for al3 you would simply name it aluminum chloride so what you need to know um for these type of problems is that whenever you're dealing with a naming if you need to name an ionic compound you should not use prefixes like mono di Tri Etc rather um you just name it aluminum chloride if it's a molecular compound where you have a non-metal combined with another nonmetal then you should use the uh prefix like monoi treton so let's go over a few problems relating to this so how would you name the compound sf6 so the first thing you need to do is you need to classify it is it ionic or is it molecular so sulfur is it a metal or a non-metal sulfur is on the right side of the periodic table the upper right side are composed of nonmetals so sulfur is a non-metal Florine is also in the upper right corner of the periodic table so it's a non-metal too two nonmetals uh means that you have a molecular compound so in this case we do use prefixes such as mono di Tri Tetra now to name it we won't say mono sulfur for the first element you don't use the word mono so you could just simply say sulfur and then for the second element you do need to use a pref so for six six represents hexa so it's going to be sulfur hexa fluide so now let's say if you want to name um this compound mg F2 so magnesium is in the left side of the periodic table so it's a metal and Florine is on the right side so that's a nonmetal a metal and a non metal um is an ionic compound so we won't use the prefixes mono Tri or Tetra so we won't say this compound is magnesium D floride that won't be correct rather the correct name is it's called magnesium the first element you just write its name and the second element you would add the end in ide so magnesium fluoride make sure you know your polyatomic ions by the way because if it's polyatomic it could have the end in i or AP so make sure you review that number three calculate the percent composition of aluminum in aluminum sulfite so the equation that you need for this problem is this the percentage or percent composition of some Element e is the mass of Element e divided by the total mass times 100% because it's a ratio if you use mass or molar mass it doesn't matter you're going to get the same answer so we're going to use molar mass in this case so the mass of aluminum we want to find the percent composition for aluminum so the mass of aluminum has to go on top we have two aluminum atoms so right now I'm just going to write uh 2 a divided by the total mass so that's the sum of all the elements so that includes the two aluminum atoms we have uh three sulfur atoms s * 3 and uh we also have nine oxygen atoms so notice that there's three oxygen here times the three on the outside and then we're going to multiply by 100% so let's plug in the numbers aluminum it's like 26.98 in the periodic table but we're just going to make it a whole number we're going to say it's 27 and then sulfur we can round it to 32 and for oxygen it's about 16 so times 100% now you can type this in all at once in your calculator um but for those of you who like to do it step by step I'll take it one step at a time so 2 * 27 that's about 54 if we add up all the numbers in the bottom 9 * 16 + 3 * 32 + 2 * 27 you should get a total of 294 so 54 ID 294 is about1 18367 and then multiply that by 100% you should get a percentage of 18 37% so the closest answer is answer Choice a now granted if we were to use exact numbers like 32.06107 ly that by 100% And you should get the answer number four nitrogen gas reacts with hydrogen gas to form ammonia calculate the mass of ammonia which is NH3 produced if 15 gram of nitrogen gas reacts with excess hydrogen gas so the first thing we need to do is that we need to write a balanced chemical equation nitrogen gas is Di Atomic so you can't write it as n it's N2 and hydrogen gas is also diatomic so make sure you know your diatomic elements N2 H2 O2 F2 cl2 br2 I2 all of those elements are diatomic and then it produces ammonia which is NH3 go ahead and take a minute to balance this chemical equation to balance it we need to put a two in front of NH3 so the nitrogen atoms are balanced we have two nitrogen atoms on both sides notice that we have six hydrogens on the right side 2 * 3 is six so we need to put a three in front of H2 so now it's balanced now this is a limiting reaction problem we have the grams of nitrogen gas and we really don't need the grams of hydrogen because it's excess it's the excess reagent The Limited reactor is the one that runs out first so this is a gram to G conversion we need to convert from the grams of N2 to the grams of NH3 we want to find the mass of ammonia and the best way to do this is to use stochiometry so let's start with what we're given 15 gram of nitrogen gas so now let's convert gr to moles whenever you want to convert GRS to moles you need to use the m Mass found in a periodic table nitrogen has an atomic mass of 14 so two nitrogen atoms would be about 28 so what this means is that one mole of nitrogen gas has a mass of 28.2 G of N2 nitrogen is about 14.01 if you want to be more precise notice that the units grams of N2 cancel so right now we have moles our next step is to use the molar ratio we need to convert from moles of N2 into moles of NH3 and to do that whenever you want to change a substance you need to use the uh coefficients in a Balan equation so what these numbers tell you is that if one mole of N2 reacts two moles of NH3 are produced so because we have moles of N2 on top top we need to put that on the bottom so for every one mole of nitrogen gas that reacts two moles of ammonia are produce in this reaction it's a one to two ratio so now the units moles of N2 they cancel and then our last step is to convert moles of NH3 into grams of NH3 so we need to use the periodic table once more so hydrogen's about 1.01 on a p act table so 1.01 * plus 14.01 NH3 has a mass of 17.04 G per one mole so the molar mass is always gr per 1 Mo so the units moles of NH3 they cancel so now we can uh answer the question so we need to type this in our calculator 15 divided by 28.2 multiplied by 2 and multiplied by 17.04 so you should get an answer of 18.24% convert it to moles and then once you have moles convert moles of reactant a to like moles of reactant or moles of product B and once you have moles you convert that to grams so these four steps will be the same anytime you need to do a gram to gam conversion so a is the correct answer uh for this particular problem number five 15 gram of sodium hydroxide is dissolved in enough water to produce a 250 milliliter solution calculate the marity of the solution now we're going to solve this problem two ways using dimensional analysis or conversions and using an equation for those of you who prefer to use formulas and equations so let's start with dimensional analysis now what we need to know is that marity is defined as the moles of solute ided by the lers of solution so when you're using dimensional analysis you need moles on top liters on the bottom so let's start with 15 G of sodium hydroxide actually first we need to write the formula of sodium hydroxide sodium is in group one of the periodic table so as an ion it has a plus one charge hydroxide is O with a negative one charge because these two they have the same charge the magnitude of the charges are the same even though the sign is opposite whenever you have two ions with the same charge you can simply pair them together so the formula of sodium hydroxide is simply NaOH now keep in mind let's say if it was like magnesium hydroxide magnesium would have a plus two charge because it's in group two of the periodic table and hydroxide has a minus one charge so if the magnit two of the charges are different you need to use this crisscross method and then the formula would be mg2 because if you mess up in the formula the whole problem is gone that's it you won't get it right so make sure you write the correct formula before you proceed so let's begin we have 15 G of NaOH and the first thing we're going to do is convert it to moles so we need to find thear mass of sodium hydroxide so we need to add up na plus o plus h in a Periodic Table Na has an atomic mass of about 23 and for oxygen it's about 16 and hydrogen's about 1 so if we add 23 + 16 + 1 the mol mass of NaOH is about 40 or 40 G per mole so because GRS is on the top left we need to put it on the bottom right so we're going to put 40 G of Na on the bottom and one Mo of NaOH on top so notice that the units grams of NaOH cancel so now we have moles now since we want to find marity we want to keep moles on top we don't want to cancel it so the last thing we needs to do is divide by the liters now we have the volume in milliliters if you want to convert Mill milliliters to liters a quick way to do it is to divide by a th000 so 250 milliliters if you divide it by th000 is equal to .25 L you can divide it by TH or you can move the decimal two unit three units actually to the left so if you move it three units to the left you get 0.25 lers so we're just going to divide it by 0.25 lers and then this is going to give us our answer so notice that we have moles on top liters on the bottom thus we have the marity so 15 / 40 and then take that result ID 0.25 and your answer should be 1.5 so we have a 1.5 molar solution now I'm going to calculate the same answer using formulas and equations so the first equation that you need to know is that uh mass is equal to the moles times the molar mass so whenever you need to find mass molar mass or moles you can use this equation so we know what the mass is the mass of sodium hydroxide is 15 um we need to find the moles here's why the second equation that we're going to use is this one marity is equal to the uh moles divided the volume in liters so in the first equation we need to solve for moles once we get moles we can plug it into the second equation so 15 G is equal to the number of moles which in chemistry moles is represented by the letter N think of PV nrt so I'm just going to use n for moles times the M Mass which we already know that to be 40 so the s for for n you got to divide both sides by 40 so n is 15 / 40 so the number of moles is equal to uh 375 so now that you have moles we could divide it by the volume in liters so the marity is 375 moles divided by the lers which is .25 L we've already converted the 250 milliliters into 0.25 l so you would have to do that here as well and then you get the same answer 1.5m so if you prefer to use equations uh make sure you add these two equations to your formula sheet for Gen cem as we go through um an equation for every problem that we solve make sure you write that down in your not notes so if you're taking like a final exam or if you're studying for like the AP Chemistry exam um make sure you write these equations down so they'll help you to like quickly review um for your gen cam final number six how many milliliters of water must be added to 200 milliliters of a 75 molar solution of sodium hydroxide to dilute the concentration to 25 M so this is a dilution problem and anytime you see a problem where it's like marity volume marity volume you can use the equation M1 V1 is equal to M2 V2 so the original solution has a marity of 75 so that's going to be M1 and the volume of that solution is 200 M now the new solution the diluted one we want it to have a concentration of0 25 so V2 is going to be the volume of that solution so let's solve for v2 to do that we need to divide both sides by 25 so these units those values they cancel now 0 25 goes into 75 three times so if you divide 75 by 0 25 you should get three and then 3 * 200 is 600 so the entire left side is equal to 600 so that's V2 but now you have to be careful because the question asked how much milliliters of water should be added if they were asking for the final volume 600 would be the answer but if they're asking how much water should be added you need to take the difference between 600 and 200 so you got to add 400 so now let's make sense of this problem so we know answer Choice C is the correct answer but here's the original solution and then this is going to be like the diluted solution so this is the old and this is the new one so originally the volume was about 200 and the concentration was 75 anytime you add water to a solution the concentration is going to decrease whenever you increase the volume by adding water the marity is going to go down so we know the final con the final volume is going to be 600 that's what we got from the equation if we triple the volume if you increase the volume by a factor of three the concentration should drop by a factor of three so notice that 200 * 3 is 600 but 75 ID 3 is 0.25 so if you triple the volume the concentration should decrease by a factor of three so now the question wanted to know how much water should we add um to the original solution so that we can make this new solution with a concentration of 025 we realized that we need 600 M of water we have 200 already in the solution so to go from 200 to 600 we simply need to add 400 Millers of water and so that's why that's the answer so be careful with the wording of the question if they wanted the final volume of the solution it's 600 if they want to know how much water should be added it's 600 minus 200 which is 400 number seven what is the correct oxidation state of chromium in sodium dichromate so for this particular problem we need to write an equation so we have two sodium atoms plus two chromium atoms plus seven oxygen atoms I'm going to put the O in parenthesis so it's not you don't confuse it with 70 and this should all equal to a net charge of zero if there was a net charge we would place it here but since there's no net charge in this compound we're going to make it equal to zero now sodium is found in group one of the periodic table so sodium usually has a plus one charge for chromium we need to solve it so let's call it X and oxygen usually has a negative -2 charge so 2 * 1 is two and 7 * -2 is -14 so we can add like terms 2 + -14 um that is -12 and then if we add 12 to both sides those numbers cancel so we get 2x is equal to 12 and then we need to divide both sides by two and so X is equal to 12/ 2 is plus 6 so that's the oxidation state of chromium it's plus 6 so let's go over some examples related to this so let's say if you want to find the oxidation state of sulfur in sulfur dioxide you would write an equation s+ 2 oxygen atoms equals to a net charge of zero an oxygen which is in group six usually has a charge of -2 so this is s - 4al 0 if you add four to both sides sulfur has an oxidation state of plus4 now what about phosphor in the phosphate ion p43 minus what is the oxidation state now because we have a net charge instead of setting the whole thing equal to zero we're going to set it equal to3 the net charge and typically when oxygen is bonded to an element other than Florine it usually has an oxidation state of -2 unless it's in its pure Elemental form which in this case will be zero any element in its pure Elemental form always has an oxidation state of zero so 4 * -2 is8 and if you add 8 to both sides -3 + 8 that's going to give you five so the oxidation state of phosphorus is five so here's some general rules if you have a Pure Element like H2 N2 O2 the oxidation state is going to be zero now Florine in a compound is always1 oxygen is usually -2 except when bond is a Florine or if it's in the form of peroxide or super oxide um if you hear the word peroxide oxygen has a an oxidation state of negative 1 but 99% of the time it's going to be -2 so if it's oxide it's -2 but if you hear the word peroxide it's ne1 now hydrogen has an oxidation state of plus one when bonded to a non-metal but when hydrogen is bonded to a metal it has an oxidation state of1 so here are some examples calculate the oxidation state for sulfur in this compound h204 so so we have two hydrogen atoms a sulfur and actually let's change it let's make it a little bit more interesting let's say find the oxidation state of sulfur in hso4 minus try that one so we have one hydrogen atom a sulfur atom and four oxygen atoms and the net charge is negative 1 now hydrogen which is bonded to a nonmetal has an oxidation state of one we're solving for sulfur so we're going to call it X and oxygen is usually -2 so we have 1 + x - 8 which is equal to 1 1 - 8 is -7 so if we add seven to both sides -1 + 7 x is + 6 so that's the oxidation state of sulfur go ahead and calculate the oxidation state of hydrogen in calcium hydde so if you were to write an equation we would have to set the net Tri to zero now calcium is in group two of the periodic table so it has a plus two charge and so we're solving for hydrogen in this case so since hydrogen's bed into a metal it's not going to be plus one if we move the this two to the other side if we subtract both sides by two we're going to get Negative -2 on the other side and if we divide both sides by two you can see that hydrogen has an oxidation state of negative one when it's bonded to a metal when it's bonded to a non-metal like in an acid it's plus one and the last unusual case is hydrogen peroxide this is the um one case where oxygen is not Z or -2 if you write an equation 2 H + 2 and if you set it equal to zero um you need to plug in plus one for hydrogen because it's bonded to a nonmetal so if we subtract two from both sides we get two oxygen atoms has a NE charge of -2 and if we divide both sides by two you can see that oxygen has an oxidation state of ne1 if it's in the form peroxide but in a form oxide it's -2 the only time oxygen has a positive oxidation state is when it's bonded to Florine if you solve for oxygen in this case we're going to call it X Florine always has a charge of NE 1 if it's not in its Elemental form so we have x - 2 is equal to Z and if we add two to both sides this is the one case where oxygen has a positive oxidation state so it has a positive two oxidation state in this particular example so that's just a quick review of how to find the oxidation state of elements in compounds number eight 38.6 Mill of a249 mol solution of sodium hydroxide was required to completely titrate 44.7 mL of a sulfuric acid solution determine the unknown concentration of the suric acid solution so notice that we have a marity a volume and we're looking for another marity given a different volume so you can use M1 V1 equal M2 V2 for this particular equation however it's different from the last example that we did in the last example it was a dilution problem which contained only one substance and that was sodium hydroxide or it might have been something else but whatever the substance was it only contain one substance now for this particular example we have two different substances we have NaOH and suric acid h2so4 so it's a little different but you can still use the equation so on the left side of the equation we're going to use uh sodium hydroxide and for the right side it's going to be we're going to apply the information that relates to h2so4 so M1 V1 is equal to M2 V2 however you have to take into account the molar ratio that's found in a balanced reaction of NaOH and h204 but it turns out that we don't actually have to write the reaction to get the right answer notice that hydroxide sodium hydroxide only has one hydroxide ion in it so therefore put a one in front of M1 V1 and notice that h24 has two protons two hydrogens in it so put a two in front of M2 V2 if you do that you're going to incorporate the M ratio of this reaction and you're going to get the right answer now we're going to solve this problem two ways using the equation and using dimensional analysis so let's start with the equation first so marity one for this to work we have to use the it for any o which is uh 249 and the volume is 38.6 milliliters the volume could be in milliliters or liters but they simply have to match if V1 is in milliliters V2 has to be in milliliters if V1 is in liters V2 has to be in liters now we're looking for the second marity that's the unknown concentration of sulfuric acid V2 is 44.7 milliliters so now to solve for M2 you need to multiply by. 249 times if you multiply 0249 * 38.6 you should get 9.61 one4 and then divided by 2 * 44.7 2 * 44 4.7 that's like 89.4 and if you take 96114 / 89.4 you should get the answer so the marity of h204 is 1075 or if you round it 0 108 now let's see if we can get the same exact answer used in dimensional analysis so we're looking for the concentration of h2so4 so you don't want to start with the volume of h2so4 rather start with the marity of the other substance sodium hydroxide if you start with sulfuric acid you're just going to make your life harder so if you're solving for the concentration of suric acid start with either the volume or the marity of the other substance now to use dimensional analysis we have to write a balanced reaction so sodium hydroxide reacts with h2so4 and it's going to produce this is an acidbase neutralization reaction and if you make a list of the ions that you have na has a plus one charge hydroxide has a minus one charge so in suric acid we have the H+ ion and we also have sulfate which has a -2 whenever you pair up H+ and O H minus you're going to get H2O and if we pair up um na+ with SO2 so42 minus I'm going to write over here so here's na+ and here's s so42 minus so we got to use that uh crisscross thing so it's going to be na2 so41 but you don't have to write the one so now we have our equation so now we just got to balance it so notice that we have two sodium atoms on the right side so we needed two in front of NaOH and if you have two hydroxide ions two hydrogen ions you need two water molecules now it's balanced so now let's start with the marity of sodium hydroxide when you're dealing with dimetal analysis you don't want write 249 capital M neoh rather you want to break down marity into its two units marity is moles over liters so we're going to write it as 249 moles of NaOH per one liter because that's what marity is it's the moles per one liter of solution now we're going to multiply by the volume that corresponds to NaOH and has 38.6 M because we have liters here we need to convert it into liters so to convert 38.6 milliliters into liters you can divide it by a th000 or move the decimal three units to the left in which case it's going to be 0386 lers so the units liters cancel so we have moles of NaOH now we want to change from NaOH to h204 whenever you want to change from one substance to another you need to use the molar ratio of the Balan reaction which is a 2:1 ratio so what this means is that for every two moles of NaOH that reacts one mole of h2so4 reacts with it so because we have NaOH on the top in the top left we need it in the bottom right so that these two units will cancel now we're looking for marity whenever you're look in for marity and if you wish to do dimensional analysis to find it you need to have moles on top and liters on the bottom we want to find the marity of h204 so we already have moles on top which means we don't want to cancel it on the bottom rather we want to put liters on the bottom now 44.7 milliliters if we convert that to liters if you divide it by th you should get 0.447 liters and we'll put a one on top so we have moles on top liters on the bottom that's going to give us marity so now let's type in what we have so 249 * 0386 / two enter get that result and then divide it again by 0447 and then you should get answer Choice a which is 01075 so now you know how to solve this problem both ways using the the equation if you think that's easier or if you prefer a dimensional analysis number nine a 250 ml sample of argon gas has a pressure of 1.25 ATM at a temperature of 300 Kelvin calculate the new pressure if the temperature is increased to 500 Kelvin and if the volume is decreased to 100 milliters so look at the units that you have and that can help you to determine what equation you need so we have a volume a pressure and a temperature and we have we're looking for a second pressure we have a second temperature and a second volume so what equation has a p a V and a t in it and another P another V and another T in it it's probably this one P1 V1 / T1 is equal to to P2 V2 over T2 the combined gas law so let's plug in what we have P1 that's the first pressure that's going to be 1.25 ATM the first volume is 250 milliliters and the first temperature is 300 Kelvin in this equation um volume could be milliliters or liters but the units have to match if V1 is in milliliters V2 has to be in milliliters pressure could be in ATM it could be in T it could be in Millers Mercury but it has to match as well so if P1 is in tour P2 has to be in tour as well however the temperature has to be in kelvin it cannot be in celsius if you put it in Celsius you will get it wrong so don't do that P2 um the second pressure we're looking for that uh the second volume that's going to be 100 m and the second second temperature is uh 500 Kelvin so what we're going to do is cross multiply so if we multiply the 1.25 the 250 and the 500 we should get a big number 15625 that does not look like a two 156 250 and then if you cross multiply for the other side um P2 * 100 time 300 300 time 100 that's basically 30,000 you just do 3 * 1 and you add the four zeros and then times P2 so now we have to divide both sides by 30,000 so the second second pressure has a value of 5.21 ATM and that's the end um that's it that's all you got to do for this particular problem so D is the correct answer Choice number 10 calculate the density of oxygen gas at STP STP stands for standard temperature and pressure at STP the temperature is 0° C or 273 Kelvin and if pressure is 1 atm but also something else that you need to know at STP is that one mole of a gas could be any gas occupies the volume of 22.4 L with that knowledge we can get the answer oxygen is diatomic it's O2 and it has a molar mass of 16 * 2 which is 32 G per mole look at the units notice that the units for our answer is like grams per liter so with these two numbers we can get the answer so start with 32 G of oxygen gas per one mole that's the mol Mass that's what we have here and then convert um moles into liters so notice that using this molar mul to ler ratio we can say that one mole of oxygen gas occupies a volume of 22.4 lit so the units moles cancel and we have our desired units which is grams on top liters on the bottom so to find the density of a gas at STP all you need to do is take the molar mass and divide it by 22.4 so 32 / 22.4 is about 1.43 and so that's the density of oxygen gas at STP now there's another equation that you could use density is equal to the pressure time the M Mass / RT let's say if it wasn't at STP but let's solve this problem at STP the pressure is 1 atm if it's different then just plug in a different pressure the mol mass is 32 G per mole that's for oxygen and then R is the gas constant so we're going to use 0 8206 um that particular gas constant has units lers time ATM over moles time Kelvin there's another gas constant which is like 8.3145 but that has a different units it has Jews moles and Kelvin um if your pressure is an ATM you want to use this one notice that the units ATM cancels and moles cancel as well the last thing we need to plug in is the temperature at STP the temperature is 273 Kelvin so the units Kelvin uh cross out as well so the only unit that we have now that's left over is um it's grams over liters and so that's going to give us the density so if you type this in the calculator 1 * 32 / 8206 and then take that result divided 273 you should get 1.43 G per liter by the way be careful with the way you type this in if you're not getting the answer when you divide by both numbers in the bottom put this in one big parenthesis because sometimes if you let's say if you were to write it like this in your calculator 1 * 32 / 8206 * 2 73 your calculator is going to divide by this number and then multiply by 273 so that's a common mistake that students make so make sure you put this in parenthesis and you should get the right answer number 11 calculate the partial pressure of ammonia if 24 G of nitrogen gas reacts with excess hydrogen gas at 298 Kelvin inside a 2.5 L container so notice that we have three different substances ammonia nitrogen gas and hydrogen gas so we need to write an equation nitrogen gas is N2 hydrogen gas is H2 and ammonia is NH3 so to balance it let's put a two in front of NH3 and a three in front of H2 now our goal is to find the partial pressure of NH3 we have the temperature and the volume what we need is the moles of NH3 but we have the GRS of a different substance N2 so we need to convert grams of nitrogen gas into moles of NH3 so let's start with 24 G of N2 over one so to convert from grams to moles you need to use the M Mass found on a periodic table the M mass of nitrogen is about 14.01 so for two nitrogen atoms it's going to be about 28.0 2 G per 1 mole of nitrogen gas now our next step is to use the M ratio the mol ratio between N2 and NH3 is a 1 to2 ratio so for every one mole of N2 that reacts 2 moles of NH3 are produced so notice that the units grams of N2 cancel as well as the units moles of N2 cancel so we have our desired units moles of any stream so now we need to take the calculator so 24 / 28.2 * 2 you should get 1713 moles of NH3 so now that we have the moles of nry we can we can now find the partial pressure of nry so we have moles temperature volume to find pressure we need to use the idog gas law PV equals nrt so the volume has to be in liters it's 2.5 n is going to be 1713 R is the gas constant 8 206 whenever you use this gas constant your volume has to be in liters it can't be in milliliters and your pressure has to be in ATM it can't be in t or millimeters Mercury and the temperature has to be Kelvin so to get the answer here's what you need to typee in your calculator it's 1713 * 8206 * 298 take that result and then divide it by 2.5 so the answer that I have um is 16. 76 ATM which rounds to about 16.8 so answer Choice C is the correct answer number 12 4. 722 G of an unknown gas is collected over water inside a 2.75 L container at 298 Kelvin the total pressure inside the container is 749 T and the vapor pressure of water is 23.7 6 T determine the identity of the unknown gas anytime you're trying to find the identity of the unknown gas um what you need to do is uh calculate the molar mass and then you want to find The M mass of every answer choice so for hydrogen the M mass is two for cl2 the M mass is 12 + 16 + 16 that's 44 for N2 it's 28 and for Xenon I think it's like 130 something I have to look at my periodic table and I don't have that with me but it's somewhere it's very high so if I get a number that's 100 something it's probably going to be Xenon but the goal is that we need to find the molar mass and um we can identify the compound now to find the molar mass we need to take the grams which we already have which is 4722 and then divide it by the moles to find the moles we can use PV equal nrt but we have to be careful because there's a lot of vapor um in this problem we don't only have one gas there's two Vapors that we have we have the unknown gas which I'll call partial pressure of the gas plus the partial pressure of water anytime you have um a collected over water problem you have to consider the partial pressure of water and this is going to equal the total pressure the total pressure we know it to be 749 T we need to find the partial pressure of the gas and the vapor pressure of water is 23.7 so let's subtract 7 49 by 23.7 so the partial pressure of the gas is 7254 T in order to use the ideal gas law pval nrt we need to convert that to ATM so divide it by 760 to convert it to ATM so this is equal to 9542 um ATM so now we can use the formula uh PV is equal to nrt so let's plug in the partial pressure of the unknown gas which we know to be 9542 the volume we have a 2.75 L container our goal is to find n once we find the moles we can find the M Mass R is going to be the same 8206 and the temperature is uh 298 k now to get the answer this is what you need to do um to get the moles multiply 9542 by 2.75 and you should get 2.62 then divide that by 0 8206 and then you should get 31.97113 moles now to find the M Mass which I'll write as capital M that's not marity by the way molar mass is going to be the grams divided by the moles it has units grams per mole so let's take the mass which is we have 4722 G of the unknown gas and let's divide it by the moles. 1073 so 4722 ID 0.173 we get a molar mass of 44.0 which means that the unknown gas is carbon dioxide because it has a m mass of 44 and that's what you need to do anytime you need to identify the identity of an unknown gas find a m Mass take the grams divided by the moles and that's it number 13 which of the following statements is not correct so this is like a true or false problem so a the average kinetic energy of a sample of gas is dependent on temperature that is a true statement um kinetic energy is equal to or proportional to 3 over2 RT that's for a sample of gas so as the temperature goes up the average kinetic energy of a sample of gas will also go up they're proportional now looking at D the average velocity of gas particles is dependent on temperature is that true or false it turns out that's another true statement the root mean Square velocity of a gas is equal to theun of 3 RT / the M Mass where the M mass is in units not in grams per mole but in kilogram per mole and R for both of these equations is not 8206 but it's 8.3145 Jew per mole um per Kelvin so as you can see as temperature goes up the root mean velocity will go up which means that the particles will be moving faster at a higher temperature so A and D are true statements so we're down between B and C now looking at C heavier gas particles exert a greater pressure on the walls inside of the container do you think that's true or false it turns out that statement is false we're going to consider two reasons for that let's kind of jump into physics now you might be thinking well pressure is force divided by area so the larger the force the greater the pressure right and that is true but if you have to realize that heavier gas particles they move slower and even if you increase your mass but decrease your speed it turns out that the pressure will be the same if you look at if you go into physics and if you look at the impulse momentum theorem the force multiplied by the contact time equals mass times the change in velocity so assuming that the time the contact time is the same for like a heavy and a heavy gas particle and a light gas particle if we increase the mass and decrease the velocity the there's going to be no effect the impulse the force multiplied by the contact time is going to be the same so you won't have a greater force if the contact time is the same so therefore heavier gas particles do not exert greater pressure now another way you can see this is to use the equation uh PV equal nrt let's consider two sample of gases let's say we have um hydrogen gas which is a very light gas and let's use a heavy gas like chlorine now let's say that they're at the same temperature let's say 300 Kelvin and we have the same quantity of gas uh one mole so one mole of chlorine gas one mole of H2 gas and let's say the volume of the container is the same let's say we have a 2 L container will the pressure be the same or different if we have a lighter gas versus a heavy gas now looking at this equation PV equal NT we have the same volume which is 2 2 L same number of moles which is one R is going to be the same it's a constant and the same temperature so if the volume um pressure I mean if the volume temperature and moles are the same based on the ideal gas law p has to be the same as well regardless if one gas is light or the other gas is heavy because molar mass is not part of this equation um the pressure is going to be the same for the Light Gas and the heavy gas so therefore heavy heavier gas particles do not not exert a greater pressure so C is the false statement therefore we're looking for the statement that is not correct so C is the answer now B is a true statement the pressure inside a container is dependent on the total number of moles of gas as we can see moles is part of the equation if we increase the moles of the gas keeping everything the same the pressure is going to increase so B is the true statement so the answer for this one is c c is the false statement number 14 which of the following conditions will allow a real gas to behave more like an ideal gas is it high temperature low temperature high pressure low pressure what would you say now let's start with a liquid if we have a liquid what's going to happen if we decrease the temperature if we put it in a low temperature environment like a freezer what's going to happen eventually the liquid will become a solid and so low temperatures do not favor the formation of gases but what's going to happen if we heat the liquid let's say if you were to boil water water is going to vaporize into steam and so it's going to turn into a gas as you can see high temperatures favor the formation of a gas so therefore a real gas will behave like an ideal gas at high temperature so we can eliminate the answer Choice C and D now in the liquid the molecules are relatively close but not that close in a gas the particles are very far apart from each other and in a solid the molecules are very close together so let's say if we have a container with gas now let's say if we increase the pressure by decreasing the volume of the container what's going to happen so if we compress the container or compress the gas will the gas turn into a liquid or will it like become more gas-like what would you say as you can see because the gas particles are more compressed they look more like a liquid so high pressure doesn't favor the formation of a gas if you increase the pressure it's going to go this way high pressure favors the formation of a liquid if you compress the gas it eventually convert to a liquid um low pressure favor the formation of a gas so a real gas will behave more more like an idog gas at high temperatures at low pressures the answer is a so let's review let's say if you have a solid if you add heat to it it's going to go from a solid to a liquid and if you add more heat it's going to go to a gas so therefore high temperatures favor the formation of gases so real gas behaves as an ideal gas at high temperatures now if we were to increase the pressure in increase in the pressure will compress a gas into a liquid and if you add more pressure it can go down to a solid so high pressure favor the formation of solids but low pressure favors the formation of gases if you decrease the pressure the solid will eventually become a gas another way you can see this is by looking at the um the phase diagram which usually looks something like this so this is the solid this is liquid this is the gas and on the Y AIS we have pressure on the x axis we have temperature so let's say if we start from here if we increase the temperature we're going to go from a solid to a liquid and eventually from a liquid to a gas so high temperature is favor the formation of gases you can see the gases they're on the right side now let's say if we started from this point here let's extend this line further let's say for to increase the press that means we have to go up the Y AIS so if we increase the pressure the gas will eventually become a liquid and then the liquid will eventually become a solid so high pressures favor the formation of solids high temperatures favor the formation of gases so if you want to get a good gas or if you want a gas to behave like an idog gas you need a high temperature and a low pressure number 15 how much energy is required to heat 75 G of water from from 25° C to 74° C and a specific heat capacity of water is 4.184 Jew per G per Celsius so you need to know the right equation to use and it's qal MCAT or MC delta T Q represents the heat energy m is the mass in grams C is the specific heat capacity and delta T is the change in temperature final temperature minus initial temperature so let's plug in the numbers that we have so the mass is 75 G the heat capacity is 4.184 and the temperature difference final minus initial 74 - 25 so 74 - 25 that's 49 and then we're going to multiply that by 4.184 and 75 and you should get 15, 376 now what are the units um for this number is it in Jews or kles now Q is in Jews because the specific heat capacity is in Jews if you look at the units the mass is in grams the specific heat capacity is jewles per gram per Celsius and the temperature is in Celsius the temperature can also be in kelvin by the way for this particular equation the unit Celsius will cancel and GS will cancel and so Q is in Jews but notice that the answer is in kles so to convert Jews into kles divide by th000 so you should get 15.4 K so answer Choice D is the correct answer number 16 how much heat energy is required to melt 25 gram of ice the heat of fusion for ice is 6.01 KJ per mole now the equation for this type of problem whenever you have a phase change there's two types it can be moles times the enthropy of fusion if it's in kog per mole or it could be mass times the enthropy of fusion if the units are like Jewels per gram or something so it really depends on the unit what we can do in this problem is we can simply convert grams to moles and moles to kles and I think that's the best way to solve it so let's start with 25 G of ice and let's convert it into moles now water has a m mass of approximately 18 hydrogen is one there two of them so that's two and oxygen is 16 so that's mol mass of 18 so we have 18 G of water per one mole so the units grams uh cancel and next we're going to convert moles into kilj using this number so this number means that for every mole of ice that you melt it requires 6.01 K so the units moles cancel and so now we can find the answer so it's going to be 25 / 18 * 6.01 and you should get 8.34 7 k now is the answer positive or negative so as you go from a solid to a liquid as you melt ice into liquid water is that an endothermic process or an exothermic process well you have to add heat to heat um to melt ice so because the Ice Cube has to absorb heat it's an endothermic process and for an endothermic process the enthalpy is positive so this is going to be a positive answer the Ice Cube needs to absorb 8347 K of heat to melt into liquid water and reverse is true if you want to freeze liquid water into ice you have to remove heat so that would be an exothermic process so freezing is exothermic and melting is an endothermic process so for this problem the correct answer is answer Choice B number 17 which of the following represents the phase change from a gas to a solid now this is something you have to know um this the answer for this is deposition so let's review all the different phase changes so from a solid to a liquid it is called melting think of an ice cube turning into liquid water it melted now from a liquid to a gas this is known as vaporization and from a solid directly to a gas this is called sublimation melting vaporization and sublimation as you go from a solid to a liquid to a gas uh these processes are endothermic the enthalpy will be positive so now let's say if you wanted to go from a liquid to a solid what happens when you put a cup of water in the freezer it freezes so a liquid to a solid is known as freezing and a gas to a liquid is known as a condensation and gas to solid is called deposition so as you go from a gas to a liquid to a solid all three of these processes are exothermic so the enthalpy is negative so energy is released for an exothermic process number 18 calculate the enthalpy of the combustion of ethanol using the following information so to find the enthropy of the reaction we need to use this equation Delta H is products minus reactants so on the product sides we have two carbon dioxide molecules and three H2O molecules minus the reactants which are C2 H5 o plus 32 so now we can plug in what we have so we have two carbon dioxide molecules each with a value of 393.15 .9 now the value for ethanol is 277 and oxygen is a Pure Element in its standard state so it has a enthropy of formation of zero so now let's plug these numbers into the calculator so 2 * - 393.7 that is equal to 787 and then 3 * - 285.0 that's about 8577 and then here don't forget to distri distribute the negative sign so it's going to be plus 277 so if we add these three numbers you should get we should get a total of -3 67.7 the closest answer uh that we have to it is answer Choice a so a is the correct answer for this problem number 19 calculate the amount of heat energy released if 10.5 gam of propane reacts with excess oxygen according to the following chemical equation so this equation is already balanced so we just need to find the energy in kles and we're given grams so here we have a thermochemical equation and in such a problem you can convert grams to moles moles to kles so let's start with the 10.5 G of C3 h8 whenever you need to convert gram to moles you need the M Mass so three carbon atoms carbon's about 12 so 3 * 12 that's 36 and plus eight for the eight hydrogen atoms so propane has a molar mass approximately about 44 so 44 gr per one Mo so the grams of C3 h8 will cancel now in a thermochemical equation you can convert moles into kles by using this number with the balance equation now since we want to convert moles of C3 h8 into kles we need to use this coefficient if we had oxygen we would use five if we had CO2 we would use three so 2200 K will be released for every one mole of propane that Bur in this reaction so we get this number from the balanced chemical equation so now we just got to plug it in so it's going to be 10.5 ID 44 * 2200 so the answer that I got is uh 525 K and it's negative so the answer is answer Choice a now granted if I were to use exact numbers like carbon is really 12.01 the answer would be closest to 530 but if you use rounded values you can still get the right answer if the choices are not too close from each other so that's it that's what you got to do for this problem number 20 calculate the energy of a photon that has a wavelength of 451 nanom there are two equations that you need the energy is equal to Plank's constant times the frequency and the wavelength times the frequency is equal to the speed of light so first well actually we can combine these two equations so if you solve for V the frequency is the speed of light divided by the wavelength so we're going to replace the V with that so to find energy directly from wavelength you can use this equation it's e equal H time C / Lambda so now let's uh plug it in so the energy is going to be planks constant 6. 626 * 10 -34 times the speed of light which is 3 * 10 8 m/s all divided by the wavelength now the wavelength is in nanometers so you got to convert it to meters if it's in nanometers this is all you need to do rewrite 451 but add * 10^9 so now it's meters and then once you type this in you're going to get the answer so it's going to be we're just going to plug it in the way it is and you should get 4.4 08 * 1019 jewles if you typed it in correctly so the correct answer for this problem is answer Choice B number 21 what is the ground state electron configuration of Florine now if you have a multiple choice test the quickest way to find the ground state or the right answer is um to use the atomic number the atomic number of Florine is nine so for the electron configuration the exponents must add up to nine looking at answer Choice a it's 1 S2 2 S2 2 P3 if we were to add the exponents 2 + 2 + 3 we would get seven so it's not a looking at B if we add 2 + 1 + 5 that would give us 8 so it's not b and for C which is 1 S2 2 S2 2 P5 if we add up 2 + 2 + 5 it gives us a total of nine c is the correct answer Florine has this electron configuration number 22 which of the following four sets of quantum numbers correspond to the last electron found in nickel well the first thing we need to do is find the electron configuration of nickel so nickel based on the periodic table has an atomic number of 28 now we're going to write out the Su levels so we can write the electron configuration for nickel so first you have 1 s then there's 2 S 2 p 3 S 3 p 3D the third level has three sublevels and the fourth level has four 4S 4 p 4 d 4f and hopefully we don't need to go past that so s can only hold two electrons P can hold six D can hold 10 F can hold 14 now we need to write the electron configuration until it adds up to 28 so first we have 1 S2 we're going to add the exponents after 1s we have the 2s suev so 2 S2 and then after 2 p i mean after 2s is going to be 2 p then 3s P can hold up to six so it's going to be 2 P6 3 S2 if we add the exponents right now 2 + 2 + 6 + 2 right now we have a value of 12 so after 3s we have 3 p and then 4S so 3 p 6 P can hold up to six 4s2 after 4S we have the 3D level so right now we have a total of 20 2 + 2 + 6 is 10 and 2 + 6 + 2 is 20 so we need eight more and the d sub level can hold up to 10 so the last one that we need is 3d8 if you add all the if you add all the electrons it's uh 28 at this point so that is the electron configuration for nickel the last the very last electron in the nickel atom is the 3d8 electron so we need to find what the four quantum numbers are for that 3d8 electron so focusing on the three n is three whatever that first number um that we have uh that's the N value for the S Sub Lev L is zero for p l will always be equal to one for the D suble L is 2 and for f l will be three so because it's 3D we know L is 2 so we can eliminate C and we can eliminate B so we're between A and D now looking at a and d the ml value is both zero but the MS value is different negative half versus posi half the answer for this problem is going to be a but I'm going to show you why it has a spin of negative half so for the 3d8 electron we said that uh n was equal to three because of this number and for the D suble um L is equal to two now when L has a value of two ml can be anywhere from -2 to 2 the D subl has five orbitals the first one has a value of -2 the second one is 1 0 1 and two so we need to fill up eight electrons since we have eight in a d level so we're going to start with the up arrows so 1 2 3 4 5 6 7 8 this is the eth electron now notice that it's in the zero orbital so that means ml is equal to zero and also notice that we have a down arrow for a down arrow the spin is minus a half and that's why the answer was a but let's go over some examples so let's review a few things so keep in mind whenever you have the S Su L is equal to zero and S only has one orbital so that means ml is also equal to zero for the P Su L is one and whenever L is one um ml could be three values P has three orbitals and ml can be -1 Z and one we talked about D already it has five orbitals so for the d sub Lev L is two and for the five levels ml can be anything from -2 to 2 and the last one is the F subl where L is three whenever L is three ml be anywhere from -3 to 3 the fs level has um seven orbitals so let's say if you want to find the four quantum numbers for the uh let's say the four P2 electron so n is the first number that you see four for the P sub level L is going to be one and the P Su L has three orbitals if L is 1 ml could be 1 Z or 1 so those are the ml values and because we're focused on the second electron we're going to draw just two arrows so the second electron is in the orbital where ml is equal to zero and because it's an up orbital I mean an up Arrow the spin is positive2 so let's look at another example let's say if we have uh the 4 f13 electron using the information that we just considered go ahead and find the four quantum numbers that corresponds to this electron so n is four for the f subl l has to be three now for f there are three or I mean seven orbitals actually with a value ranging from -3 to 3 and a 13 electron lands in let's see one two 3 four five 6 7 8 9 10 11 12 13 so here's the 13th electron it's any orbital that has a value for ML which is two and because it's facing in a downward Direction the electron spin is minus one2 so that's just some background information on common numbers and uh how they work number 23 which of the following four sets of quantum numbers are incorrect is it A B C or D typically the first thing I look at is the electron spin the electron spin can only be either plus or minus a half if you see any other numbers other than this fraction you know it's wrong so if you were to see msal 1 you could eliminate that answer so looking at the spin for all of them it's a fraction plus or minus a half so all of those numbers are acceptable the second thing you can do is compare n and L now L has to be less than or equal to nus one so what this means is that if n is four L cannot equal 4 or anything more than four if n is three L can equal two or anything less than two it could be two one or zero but it can't equal three or four so looking at a n is 3 l is 2 L has to be at least one less than n so a is okay same is true for c for B if n is four L is zero and D if n is four L is three as long as n is greater than L at least by a factor of one um it's okay okay now the last thing we need to look at is the the ml values the orbitals the or the magnetic quantum number ml has to be between L and L so what this means is that let's say if L has a value of two that means ml has to be between -2 or 2 so looking at D if L is 3 ml could be anywhere from -3 to 3 that means it could be -3 -2 Nega 1 0 1 2 or 3 and ml is negative 1 so that's okay so D is fine looking at um let's see answer Choice a if L is 2 ml could be zero because ml could be -2 1 0 1 and 2 same is true of C the only that doesn't work is B when L is zero ml can be only one value zero it can't be negative 1 so B is the one that's incorrect so just to review whenever L is zero ml can only be zero this is the S suev and there's only one orbital so ml cannot equal 1 now if L were to equal one this would be the p sub level which has three orbitals and ml could be 1 0 or 1 and so this would have worked if it was if L was one but since L is zero ml can't be negative 1 number 24 how many orbitals are in the N equal 4 principal energy level the equation that you need is this the number of orbitals is equal to n^2 so that's 4^ SAR which is 16 so D is the correct answer now you might be wondering okay why does that equation work in the first energy level where n is equal to 1 all we have is the 1s level and the second energy level when n is two we have the 2s subl and the 2p suel when n is three we have the 3s level and the 3p suble and a 3D for the fourth eny level we have uh 4S 4 P 4D and also 4f now the foure level can hold well let's start with one the firste level can hold up to two electrons and for every two electrons um you only have one orbital so the 1s level has only one orbital now we know that s has one orbital but P has three orbitals which is a total of four orbitals in the second energy level notice that the total electrons that we have is eight and if you divide eight by two you get four because there's only two electrons that can be in a single orbital for the third energy level if we add up the orbitals s can hold one p can have three orbitals D has five that gives us a total of nine and for the fourth en level if you add up all the electrons you should get 32 half of that is 16 s has one orbital P has three orbitals D has five F has 7 if you add those numbers you get 16 so notice that the number of orbitals is equal to n^2 1 squ is 1 2 squ is 4 3 squ is 9 4 squ is 16 and that's how it works but if you want to see it visually the Force Level has only one orbital the 4 P level has three 4 d has five and 4 f F has seven so therefore we have a total of 16 orbitals in the four enage level by the way the video that you're currently watching is the first 2hour trailer version of a longer 8 hour video so this video currently has only 40 multiple choice problems but the the entire video has uh 160 multiple Choice problems so I'm going to post a link um which you can access the entire video and any topics that we missed in his trailer version you can find it in the other video for the most part so I'm going to post that link and then you could check it out when you get a chance number 25 which of the following choices correctly ranks the different forms of electromagnetic radiation in order of increase in wavelength so you need to know it in this order all the way to left we have radio waves after which we have uh microwaves and then infrared radiation and then visible light so after infrared it's red light orange or Ro Biff yellow green blue and then Violet after Violet next comes ultraviolet radiation and then you have xrays and then finally after xrays gamma rays to the left radial waves have the longest wavelength I'm going to put the lamba symbol for wavelength and to the right gamma rays have the most energy and they also have the highest frequency so these are some things that you want to know with regard to electromagnetic radiation So based on a question we want to rank it in order of increase in wavelength so radio waves have the longest wavelength and gamma rays have the shortest so looking at that we could see it's answer Choice a gamma rays have the shortest wavelength next x-rays after xrays is ultraviolet and then after ultraviolet it's visible light so out of those for visible light or light waves they have the longest wavelength gamma rays have the shortest wavelength and that's it for this particular problem number 26 which of the following elements has the highest first ionization energy is it helium Florine oxygen or chlorine ionization energy is the energy that is required to remove a veence electron from a gaseous atom it could be just the outmost electron sometimes it could be a core electron dependent on if it's like the second or third ionization energy but it's the energy required to remove the outermost electron which is usually a veence electron so ionization energy increases as you go this way in a periodic table so it increases as you go up a column or to the right now oxygen so on a peric TP we have carbon nitrogen and then oxygen and then Florine and Below Florine we have chlorine and then here's like neon and then helium is over there so notice that helium has the highest ionization energy compared to oxygen Florine and chlorine helium is to the right of all the elements and it's above it as well so if we were to rank it helium would have the highest and then the next one would be Florine and then after that we have oxygen and and chlorine so the correct answer for this problem is answer Choice a by the way just to review the other Trends Atomic radi or atomic size increases as you go this way um in the periodic table so as you go to the left and down it increases electro negativity increases towards Florine Florine is the most electronegative element and metallic character increases as you go to the left and down so those are some other periodic trends that you want to uh be familiar with number 27 which of the following molecules has a trigonal plan or molecular geometry is it A B C or D so let's draw each one let's start with sulfur dioxide sulfur has six valence electrons and oxygen also has six but we have two of them so there's a total of 18 now for molecules that do not contain hydrogen there's a way that you can calculate the number of Lone pairs or electrons on a center atom if you take 18 and subtract it by the highest multiple of eight just under 18 um you'll get the number of electrons on the center atom so multiples of eight are 8 16 24 32 but the highest of this that's under 18 is 16 so if we take 16 from 18 we get two what this tells us is that sulfur has two electrons or one Lo here on it and it also has two oxygen atoms now sulfur can have what is known as an expanded octet it turns out that elements that usually have expanded octets like third row elements like phosphorus sulfur and chlorine the number of bonds and dots that are on that element is is usually equal to the valence electrons so because sulfur has six valence electrons it's going to have a total of six Bonds in dots so since already has two dots it's going to have four bonds so one two 3 four the exception to this is if sulfur has a formal charge now whenever oxygen has two bonds it requires two electrons or two lone peirs I meant four electrons two lone pairs so notice that every oxygen atom has eight electrons if you add up all the electrons you should get a total of 18 and that's the L structure for sulfur dioxide and it has a bent molecular geometry now CH4 um for most carbon hydrogen compounds we really don't need to add the electrons but if you wanted to uh carbon has four valence electrons hydrogen has 1 * 4 we should get a total of eight hydrogen can only form a single Bond so this is the only only way we can draw it and as you can see each Bond represents two electrons so we have a total of eight electrons methane has a tetrahedral molecular geometry and keep in mind for a tetrahedral structure the bond angle is about 109.5 for so SO2 is less than 120 now BF3 boron has three valence electrons Florine has 7 * 3 24 24 is a multiple of eight so that tells us that there are no lone pairs on a central Boron atom that multiple of eight rule works pretty well if there's no hydrogens involved because hydrogen can only have two electrons and not eight now Florine wants to have eight electrons each Florine atom has one Bond so we need to add three lone pairs so that every floring atom has eight electrons 8 * 3 we get 24 that's the L structure of BF3 and and it is the correct answer it has a trigonal plane of molecular geometry and a bond angle of 120 now for NH3 um the multiple rule of aan doesn't work whenever you have hydrogen um nitrogen has five o electrons and hydrogen has one * 3 so we get a total of eight now hydrogen can only form one Bond so right now that's six electrons to get eight we need to add a lum here so that's the L structure for NH3 and it has a trigonal pyramidal structure with a bond angle of 107 so it's not trigonal planer but trigal pyramidal so C is the correct answer for this problem number 28 which of the following molecules has sp3 hybridization around the central atom is it water BF3 brf3 or sulfur heop floride let's start with sf6 let's draw the Lis structure structure sulfur has six veence electrons and Florine has 7 * 6 which is 48 now because 48 is a multiple of eight there are no lone pairs on the central sulfur atom so sf6 looks like this and each Florine atom has three lone pairs so if you add up the total electrons you should get 8 * 6 which is is 48 the molecular geometry for this compound is octahedral the bond angle is about 90° all around and the hybridization is S1 P3 D2 notice that the exponents add up to six 1 + 3 + 2 is 6 because the sulfur has six groups attached to it so sometimes it could be written as D2 um sp3 you might see it written that way as well now let's look at brf3 bromine has seven veence electrons and Florine also has seven time 3 so we get a total of 28 which is not a multiple of eight so we're going to subtract that number by the highest multiple of eight just under 28 so multiples of eight are 8 16 24 so if we subtract 24 from 28 we get four so what this tells us is that bromine has four electrons or two lone peirs and it has three Florine atoms Each of which has three lone pairs as well I'm not going to draw the lone pairs but just know that Florine each have three L pairs so if you add up all the electrons you should get a total of 28 including the three lone pairs on each flooring atom now now the molecular geometry for this compound is it has a t-shape and if you were look at the hybridization bromine has five groups it has three Florine atoms and two lone pairs so for five groups the hybridization is uh S1 P3 D1 which adds up to five so it's not C and it's not D now looking at B we know that Boron Tri fluoride uh looks like this because we drew it in the last example and so boron has three groups so therefore it has an S1 P2 hybrization 1 + 2 is three so the answer must be water if we draw water water has eight veence electrons hydrogen has one and there two of them and oxygen has six valence electrons so for a total of eight and right now we have four electrons Each Bond represents two so we need two more lone pairs that's the structure for water it has a bent shape and notice that water has four groups it has the oxygen atom has two hydrogens and two lone peers so for four groups the hybridization at the central atom is S1 P3 1 + 3 = 4 so the correct answer is a it has an sp3 hybridization around the central oxygen atom number 29 which of the following molecules is polar is it A B C or D if we were to draw the L structure of CH4 methane this molecule is nonpolar due to the Symmetry uh that it has typically whenever you have a carbon hydrogen bond it's relatively non-polar the electro negativity value for carbon is like 2.5 and for hydrogen is 2.1 so if the electro negativity difference is less than 0.5 the bond is non-polar so a is eliminated now for carbon dioxide if we were to draw the Le structure it would look something like this it has a linear molecular geometry now the carbon oxygen bond is polar carbon has an electro negativity value of 2.5 and for oxygen is 3.5 so the difference is much larger than .5 so it has a polar bond oxygen has a partial negative charge because it's more electronegative than carbon so if you were to draw the dipole moment the arrow would Point towards the more electronegative oxgen atom but notice that these arrows are opposite to each other and therefore they cancel and so the net dipo moment for this molecule is zero therefore even though it has polar bonds overall CO2 is a non-polar molecule now for Boron Tri floride the situation is similar as well Florine is much more electronegative than Boron and so it has a partial uh negative charge and so the bonds are very polar however due to the symmetry of this molecule all of the arrows they point in opposite directions and so they all cancel out and therefore this is a non-polar molecule if you're wondering why those arrows cancel out let's go back to physics you have an arrow going this way and an arrow going this way and another one going this way now looking at the arrow on the bottom right it has two components it has a y component and an X component and the same is true for the arrow on the left it has a y component and an X component these two x components are opposite to each other so they cancel and these two y components cancel with the larger one on top so all all of it cancels and it's nonpolar the only one that is polar is na um nf3 if you draw a leou structure it has a lone here and the DI moments do not cancel let's add up the valence electrons first for nf3 nitrogen has five veence electrons it's in group 5 a of the periodic table and for Florine it has seven 7 * 3 is 21 plus 5 you get a total of 26 and if you subtract that by the highest multiple of eight just under 26 8 16 24 you get two so what that number tells you is that nitrogen has two electrons or one lone pair and every Florine atom has three lone pairs because they want to have eight electrons and so the total number of electrons in a structure is 26 we have a trigonal or trigonal pyramidal structure now Florine is much more electronegative than nitrogen it has a value of 4.0 and for n nitren is 3.0 so Florine Bears the partial negative charge nitrogen Bears the partial positive charge so if we were to draw the diap moment it would Point towards the flooring atom so notice that we have three arrows going in the for the most part the general downward Direction and so these arrows do not completely cancel for the most part they're going towards the bottom and so there's a net diap moment and also the presence of this L here also contributes to the polarity of this molecule so nf3 is a polar molecule it has a net dipole moment number 30 rank the following intermolecular forces in order of decrease in strength what we need to know is that hydrogen bonds are stronger than dipole interactions which are stronger than London dispersion forces so the correct answer is answer Choice B now you have hydrogen bonds whenever H is directly attached to nitrogen oxygen or Florine dipole interactions occur whenever you have a polar molecule so dipole dipole interactions occur between two polar molecules ldf London dispersion forces also known as Vandal forces they're found in everything but they are the most important interaction in non-polar molecules so let's say if you were to have a compound like water water has hydrogen bonds so um it has an o Bond so it has hydrogen bonds anything that has hydrogen bonds is polar so it has dipole interactions and it also has ldf which is found in everything now let's say if you have a polar substance like so SO2 so SO2 would contain dipole interactions plus it would also contain ldf but if they ask you which one is the strongest intermolecular force in SO2 you would choose Dion of actions for water you would choose hydrogen bonds now let's say if you have like methane anytime you have a substance that contains only carbon and hydrogen bonds is non-polar the only intermolecular force that's found in methane would be like ldf dispersion forces or vandall forces number 31 which of the following molecules has the highest boiling point now if we look at a we have a compound that is completely nonpolar it only contains carbon and hydrogen bonds so when it's non-polar the only force that it has is London dispersion forces now compound D is also non-polar and so it has Lon dispersion forces only so therefore these two which contain the weakest of the intermolecular forces they're going to have the lowest boiling point so we can eliminate answer Choice A and D now B we have a sulfur and if we were to draw the Le structure there's a sulfur group a hydrogen and a methyl Group which is a ch3 and it has two L pair so this molecule is polar it has a bent shape similar to that of water and because it's polar it's going to have a dipole interactions so it's going to have a higher bowling Point than a or D now looking at answer Choice C methanol we have an O Bond so methanol has hydrogen bonds and hydrogen bonds are stronger than dipole interaction so therefore we should expect that methanol should have the highest Bing point because it has the strongest of the molecular forces so it has H bonds so C is the right answer number 32 25 gr of sodium hydroxide is dissolved in 300 G of water calculate the mality of a solution now mality is different from marity marity with an R is the number of moles of solute divided the lers of solution mity is the moles of solute divid the kilog of solvent so let's find the moles of solute and let's divide it by the kilograms of solvent water is a solvent it dissolves the sodium hydroxide and we have 300 gr of water so how many kilograms is that to convert GRS into kilograms you could divide by TH or move the decimal three units to the right so 300 gram is about3 kilograms let's start with 25 grams of sodium hydroxide and let's convert it to moles to convert grams to moles you need the mol Mass according to the periodic table sodium has an atomic mass of 23 and for hydrogen it's about one and for oxygen is 16 so NaOH has a mol mass of 40 gram per one mole so notice that the units grams of NaOH cancel so right now we have moles so now we got to divide it by the3 kg of solvent so whenever you have uh moles on top and kilograms of solvent on the bottom that's going to give you the mity so 25 / 40 is about 625 that's how many moles we have and if we take that number of moles divid by3 kilg we get a mality of 2.08 so D is the right answer number 33 calculate the mality of a 24% HCL aquous solution so how would you do that when you have a mass percent a 24% htl solution what you need to realize is that you have 24 G of HCL per 100 G of solution now the solution has two components the solute and the solvent the solute plus the solvent is equal to the solution now because we have an aquous solution the solvent is H2O so 100 minus 24 is 76 therefore we have 76 G of H2O now granted the ratio is what is really is important so we can also have 48 G of htl but that will correspond to 200 g of solution or we can have 12 gr of htl for every 50 Gams of solution but the best way to answer this problem is convert this percentage into grams and base it out of 100 so if you have a 30% htl solution that means you have 30 gram of htl 70 G of water and 100 G of solution if you do it that way then it's going to be easy to get the answer so now it's going to be similar to The Last Problem um that we did previously number 32 so let's start with 24 G of htl and let's convert it to moles chlorine has a mol mass of 35.45 and for hydrogen it's like 1.8 so the mol mass for HCL it's about 36.4 58 G per one mole so these units cancel and the last thing we need to do is divide by the kilograms of solvent we have 76 G of solvent to convert G to kilog divide by th000 or move the decimal 3 units to the left so 76 G is equal to 076 kg so 24 / 36.4 58 that should give you 658 moles and then divide that by 076 so the answer for this problem is 8.66 that's the mality of the solution so that's how you can convert mass percent into mality 34 determine the marity of a 27% hi solution with a density of 1.21 G per Mill so we know marity is the moles of solute over the lers of solution so if we can have the grams of solution we can use the density to get the milliliters of solution and then the liters of solution so we could find the marity but let's uh write down we have so in a 27% solution that means that there are 25 excuse me 27 G of hi for every 100 gr of solution and if we subtract the two numbers 100 minus 27 that means that we have 73 G of water but we probably don't need that for this problem since we're not looking for malaly now we can easily get moles of solute by conting of hi into moles but we need the liters of solution so 100 G of solution let's convert that to liters so the density is 1.21 G per milliliter that means that 1 milliliter of solution has about 1.21 G so notice that the GRS cancel and now we can convert milliliters into liters there's a 000 m per 1 liter so 100 / 1.21 / 1,00 we have a volume of 08 26 l so now we can calculate the marity so starting with 27 G of hi let's convert it to moles I has a m mass of 26.9 plus 1.8 for hydrogen so the M mass is 12798 now that we have moles of solute on top all we need to do is divide it by the lers of solution to get marity so 27 / 12798 you should get. 211 and then divide that by 0826 and the final answer is about 2.55 so answer Choice C is the right answer it really rounds to 2.56 but C is the closest answer number 35 determine the boiling point of an aquous solution of aluminum chloride so in this problem we have the mality we have the KB the equation that we need is the temperature of the solution minus the temperature of the solvent is equal to KB time the mality time the V half Factor you might have seen this equation as delta T equal k b * m * I just keep in mind that delta T is the temperature difference between the boiling point of the solution and the boiling point of pure water now the only thing that we need in this equation is the Vanhoff Factor what is the Vanhoff factor for this problem well look at aluminum chloride how many ions can aluminum chloride dissociate into aluminum chloride contains an aluminum plus three ion and three chloride ions so it has a total of four ions so therefore the Venta factor is four one mole of aluminum chloride will generate four moles of ions so now if we plug if we plug in the numbers that we have in this equation um this is what we have the temperature of the solution minus the temperature of pure water which is the boiling point of pure water is 100 degrees C KB that's . 512 the mality is 1.24 and the vental factor is four so now we have to solve for the boiling point of the solution so 0512 * 1.24 * 4 will give us a value of 2 5395 so now we just have to add 100 to both sides so the boiling point is going to be 102.5 if you add 100 to 2.5 so answer Choice C is the right answer so keep in mind whenever you add salt to water the boiling point goes up so let's say if you have pure water which has a boiling point of 100 once you add salt to it the boiling point could increase to like maybe 102 the freezing point of pure water is about 0 degrees celius and if you add salt to that the freezing point goes down it could be like maybe ne5 or -10 depending on how much you add so the equation that we dealt with is called boiling point elevation because anytime you add salt to water the boiling point goes up and there's another equation which is very similar it's Delta TF is equal to NE KF M * I this is called a freezing point depression because when you add salt to water the freezing point or melting point decreases it becomes less than zero and this is why um salt is added to icy roads because whenever you add salt to ice the melting point of ice decreases and so it's easy for ice to melt whenever it's salt is added to it so those are some things that you want want to keep in mind as relates to this topic number 36 32 G of an unknown non-electrolyte solute was dissolved in 345 Millers of solution the osmotic pressure of the resulting solution is 27.1 atmospheres at 298 Kelvin calculate the molar mass of the unknown solute so the important expression that we have is osmotic pressure Pi which is osmotic pressure is equal to to the marity time the gas constant time the temperature time the V Factor now in order to find the M Mass we need the grams and we need to divide that by the moles now we already have the grams it's 32 G that's the grams of the solute in order to find moles we got to find marity so we can rewrite the equation like this Pi is equal to moles over L time r T * I so our goal is to solve for the mol so let's plug in what we have the pressure is uh Pi which is 27.1 and for the moles we're going to call it X that's what we're looking for now the liters is the volume we have 345 milliliters of solution to convert that into liters we got to divide that by a th so that's equal to 345 L and then R is the gap gas constant that's 8206 the other one 8.3145 that has units Jews per Mo Kelvin but we need to use this one because it has the units ATM in it and it has liters as well the temperature is 298 Kelvin and the VHA factor for a non-electrolyte is always equal to one because non-electrolytes do not dissociate into ions so so now we need to do math to solve for x so let's multiply both sides by 345 so these numbers will cancel so on the left if we multiply 27.1 time 345 uh we should get 9. 3495 and that's equal to 298 * 8206 so that's going to give us 24453 88x so if we divide both sides by 24 45388 x is equal to 3823 moles now in order to find the molar mass we need to take the grams which is 32 G and divided by the number of moles and so we get 83.7 G per mole and that's it so D is the right answer number 37 35.1 G of sodium chloride is dissolved in 200 100 G of water the vapor pressure of pure water is 23.7 6 T at 25° C calculate the vapor pressure of the solution the equation that you need for this problem is the vapor pressure of the solution is equal to the mole fraction of the solvent times the vapor pressure of the solvent now we already have the Vape pressure of the solvent for pure water is 23 .76 but what we need is to find the mole fraction of the solvent so let's convert everything into moles so we have 35.1 gram of sodium chloride and the M mass for NAC for na it's about 23 and for chlorine it's about 35.45 so that gives us a mol mass of 58.4 G now we need to take into account that sodium chloride has a v factor of two that means there's two ions per formula unit and since vapor pressure is a colligative property which is a property that is dependent on the number of solute particles we have to find the moles of ions rather than the moles of sodium chloride so there's two moles of ions per one mole of NaCl so we have to multiply the moles of Na by two so 35.1 / 58.4 multiplied by two you should get 1.2 moles of ions now let's convert the 200 g of water into moles water has a m mass of about 18.02 G per mole so for water we have 11 one moles of H2O so now we can find the mole fraction of the solvent which is the mole fraction of water the mole fraction of water is the moles of water which is 11.1 divided by the total moles which is the moles of water plus the moles of the solute particles so the mole fraction that you should get for water is about 902 so now we can use this equation so the vapor pressure of the solution is equal to the mole fraction of water which is a solvent times the vapor press of the solvent which is 23.7 to so the answer that you should get is 2.43 tour so answer Choice B is the correct answer for this problem number 38 calculate the average atomic mass of boron if the relative percent abundance of Isotopes B10 and b11 are 19% and 8 1% respectively so what we need to do is we need to take a weighted average of the mass of the Isotopes so the equation that you need looks something like this so the weighted average is equal to the uh first mass times the percentage that corresponds to that Mass plus the second mass times its respective percentage so the first mass isotope 10 or B10 has a mass of 10 and the percentage for that is about 19% which as a decimal that's .19 you just divide 19 by 100 now the second Mass b11 has an atomic mass of 11 and 81% as a decimal is 81 so at this point all you need to do is really just you just got to type it in the calculator so you should get for your final answer 10.81 so that's the average atomic mass of boron so B is the answer number 39 15.5 G of magnesium metal reacts with excess nitrogen gas to produce 18.2 G of magnesium nitride calculate the percent yield of magnesium nitride so the first thing we need to do is write a balanced reaction so magnesium is mg and it's going to react with nitrogen gas which is N2 it's diatomic and that's going to produce magnesium nitride so magnesium nitride is an ionic compound so we can't just write mgn you have to make sure you uh write it in the correct ratio magnesium is in group two so as an ion it has a plus two charge and nitrogen is in group like 15 or 5A and it has a minus charge a minus three charge so you got to use like this crisscross method where the two is going to go here the three is going to go there and and so the correct formula is mg3 N2 so now we need to balance the reaction we have two nitrogen atoms on both sides so we don't have to worry about that all we need to do is put a three in front of mg so it's 311 so now this problem wants us to find the percent yield the equation for percent yield it's equal to actual yield divided theoretical yield times 100% now usually the actual yield is given to you if you need to find a percent yield and the yield is based on a product so notice that they mentioned or this problem mentioned that uh 18.2 grams of magnesium nitride was produced so that's the uh actual yield um for this particular reaction so we got to find a thetical yield the thetical yield is the maximum amount of mass or GRS it could be in moles too the maximum amount of product that can be produced in a reaction so we know nitrogen gas is the excess reactant that means magnesium metal is the limiting reactant so let's go ahead and solve this problem so we need to convert grams of magnesium to Gam of magnesium nitride and that's going to be the theoretical yield so let's start with 15.5 G of magnesium and let's convert it to moles in order to convert it to moles we need to use the mol mass for magnesium it's about 24.31 G so the units grams cancel so now we can convert moles of magnesium to moles of magnesium nitride so we need to use the M ratio it's a 3:1 ratio so for every one mole of magnesium nitride that's produced in a reaction 3 moles of magnesium are consumed the last step is to convert mag magnesium nitride from moles to G so we need to find the M Mass so we have three magnesium atoms which is so it's going to be 3 * 24.31 and two nitrogen atoms which is 2 * 14.01 and I'm going to use the calculator for this if I did that correctly that is equal to 10.95 so it's 10095 G per 1 mole of magnesium nitride so moles of mg cancel and moles of mg and3 cancel so now we have the grams so to calculate it it's going to be 15.5 / 24.31 take that result and then divide it by 3 and then multiply by 10095 if you did it correctly you should get this answer this is the theoretical yield of the product so that's the the maximum amount of magnesium nitride that we can get if all of the 15.5 G of magnesium metal reacts that's the most that we can produce in this reaction but now according to this problem we only got 18.2 uh grams of magnesium nitride out of the maximum 211.45 gram so this is how you calculate percent yield it's going to be the actual yield which I'm going to represent as a y divided by the thre medical yield and then times 100% so if you plug those numbers in the answer should be 84.8% so that's the answer to the problem so answer Choice C is the correct answer for this problem number 40 calculate the pH of a 5.43 * 10us 3 mol solution of barium hydroxide the first thing we need to do is find the concentration of hydroxide if we can do that we can find the PO and then we can find the pH of the solution so barium hydroxide dissociates into uh the barium plus two ion and two hydroxide ions in water barium hydroxide it's fairly soluble in water and so notice that we have a 1:2 ratio between barium hydroxide and hydroxide ions so if you wish to find a the um excuse me the concentration of hydroxide we need to multiply the concentration of Barum hydroxide by two because we have a one and two ratio but if you wish to convert it you can write it like this 5.43 * 10us 3 for barium hydroxide and then you can multiply it by the 2:1 ratio so therefore the concentration of hydroxide is about 01086 M so now we can find a p which is negative log of the hydro oxide concentration so if you type in uh negative log1 086 in your calculator you should get a a p value of 1.96 4 now another equation that you want to know is that the pH plus the P must add to 14 whenever water is a solvent so therefore the pH is 14 minus the P or 14 minus 1. N64 and that's going to give you the answer so the pH is equal to 12.04 answer a is the right answer