Transcript for:
Chemical Composition Basics

okay we're going to move on to chapter six we are done with chapter five chapter six any questions in class right now um you have what is called uh chemical composition that's the title of chat in the next chapter chapter six chemical composition so here chemical composition basically like we talks more about the compounds what do you have in a compound and then what are the formulas you write for a compound and then if you have a compound with a formula what is the mass percent of each element in a compound so on and so forth okay so the four major concept we are going to see is what is called the moon someone's audio is on please turn it off [Music] uh the mole concept and the second one is what is called the percentage composition composition the third concept we learn is something called empirical formula and then we are going to do what is called a molecular column and this chapter purely uh like pretty much the whole chapter is involving math okay so these are the four main concepts here now um the percent composition uh in your textbook it says mass percent composition okay so if i look into this chair textbook uh basically sections 26.1 to 6.4 covers the small concept six point one to six point five and then uh chemical formulas has conversion so the mass percent is covered in six point six and six point seven and then this one empirical formula is 6.8 this is 6.9 okay so that's basically how it is distributed so even though i said this is the only concept uh then remember the this goes with the four sections but the four sections are pretty much the idea is the same okay nothing nothing different they just split them into i just usually teach them with a single figure and then give you multiple problems and then they just split it into four small sections that's all it is okay and then i'm also going to work the examples for this concept from the textbook we will look into the example and then i will try to work on the skill builder problem right below each example i tend not to work on the example itself because it is posted with solution in canvas so i try to do an extra problem there so we are going to for today's lecture we are going to have more concept okay and then when i come back on monday i am going to proceed further down so what is the concept of mole the concept of mole is an easy one to understand easy to define but then i i see that more and more and more people students like they it is an abstractive idea then what they do is they get confused and then they use the wrong formula or they don't understand what the question is asking so on and so forth okay so let me give you a very generic idea of mole and then i am going to start start with a picture instead of giving a lot of formulas if you have a picture anything with the mole you basically look at the picture and what formula to use it okay now the mole concept starts in this chapter sure but then it doesn't end here the mole concept is going to continue as long as you are involving yourself with chemistry okay maybe there is one chapter where it talks about electronic configuration later you may not need more concept generally a mole concept is tightly associated with chemistry that's a very car like writing an equation for a chemical reaction in any chemistry class the same thing mole concept is something which is related to many many concepts that's why it just basically occupies the the entire space of chemistry okay very very important so to put it in a very simple word what do you mean by a mole think of mole as something like a package okay a package and i talk about more i always think a package all right a package so far what are some of the packages we know in real life something like which we deal with every uh in everyday life we we always refer to a package called a dozen it doesn't matter like uh which matter we are taking a dozen always we associate with the number 12 is that correct so one dozen is to us is always the countdown if i have a dozen eggs it is tall eggs and donuts it is 12 donuts like that it doesn't matter what the matter is one dozen we always associate with 12 we also know a packet something like a pair one pair means two we always know that right a dozen is twelve a pair is two a mole is a specific number and that number magic number is this 6.422 times 10 power 23 okay so that's the number this number has a special name named after the scientist who developed it this is called an avogadro number so avogadro number uh is something you don't have to memorize it is always given in the bottom of this periodic table right here you see that number 6.022 times 10 power 23. so like i said like we all know that one if you have a package with one dozen we know it is 12. a package with the pair means two like the same way a package of called mole as a number avogadro number you see how big your number is this six zero two two followed by twenty zeros it's a huge uh number but that's a number and named after the scientist you developed this so when i take any element in the periodic table when i say one mole say for example one mole of carbon that means what is the building block of any element atoms is that right so when you say one mole of carbon means it will have this many number of carbon atoms just like we are saying one dozen donut will have twelve donuts the same way one mole of any element will have avogadro number of atoms okay that add atoms of that element something like one mole of sodium will have avogadro number of sodium atoms one mole of potassium will have avogadro number of potassium atoms one mole of calcium will have avogadro number of calcium atoms okay so atoms are the building blocks of element the same thing i can also do for compounds one mole of your compound will have avogadro number but the base unit for compounds are molecules okay we don't see atoms for a compound the base the building block for a compound is molecules the building block for elements or atoms okay now a little bit why do i have to know all this one what is the significance of mold okay now remember uh say for example any real life example if you take coffee and then if someone asks you how much sugar you want you always say i need a teaspoon or a one teaspoon two teaspoons something like that we scoop it in a teaspoon and then have it right if you happen to have sugar now sugar is something i can see a small small granule nobody tells you like i need about five five thousand granules of sugar nobody is going to measure like that that's something you can see but you are not going to count it the same thing if you are a handyman and doll when you go and buy screws from a hardware store you buy it by a pound okay you don't really buy a single unit now these are something which we see even that we buying them in a package atoms are is something you cannot see you our eyes cannot uh you need a very special very very very special instruments to see atoms now only the recently they have developed some very sophisticated instruments to figure out an image of an atom but usually we can't see now if i supposing if i make water h2o i know i have two hydrogen atoms and one oxygen atom there is no way i can go and then take two hydrogen atoms from my lab and one oxygen atom make them combine i can't do that just like a handy person basic hardware with the with the package i have to say take a certain package of two elements make them do in a reaction okay now for me if i have to do a reaction let's say let's say i am going to set up a reaction in the lab the only way i can do that is how i just have to go and take a scale if i have carbon i can take like uh like six grams of carbon i can measure them right i can weigh them but the reactions is happening in an atomic level i do something in mass when i do the reaction i take some measurements like in masses that's a macroscopic word reactions are happening at a microscopic level atoms i know i take i have taken six grams of carbon in a reaction say for example but i don't know how many atoms it has but the reaction is happening in an atomic level how do i manage this so from the mass which is a macroscopic level to the reaction which is happening what is called in a microscopic world mole is a guy who is connecting it's more like a connecting bridge so if i take something in mass i know how many moles i can take using a calculation that mole calculation guarantees how much how many atoms i have very very important okay so mole is nothing but a connecting bridge between the mass which is the macroscopic world we do it in a lab and the reactions which is happening in a microscopic world it is just one of the mole almost mole connects so many concepts in chemistry so there used to be a time people say there was a time where rome civilizations are so famous right so they used to say something like all roads lead to rome people used to say that there used to be a time if i have to extend that terminology to chemistry all concepts in chemistry lead to more you've got to have mole is the central figure through which we can learn many many many concepts and for the rest of the semester you are going to see that mole doesn't start and end with chapter six mole is going to come with you as long as you take a mystery okay so now you know the significance of more a mole is basically i do something here and it is just like you do a computer program or something you feed some information in a computer computer cannot read your language numbers and all it converts it into a code a binary code and then it gets it to the processor when it does that it gives you the information back okay so that's basically how a computer behave they don't computers you will be surprised that they don't understand anything other than zeros and one every information we give it to a competitor it's converted into zeros and ones they're called binary digits anyway i don't want to get into the computer world a whole lot but that's basically what it is just like a computer taking your information process this it is going to process it in a different way get the information back put it to you on your screen in a way you can understand the same thing mole is doing that for us i take something in mass and then and it is happening in an atomic level mole tells me then through the mole i will know my information okay because we are connecting mole with the microscopic to the macroscopy i need to know what is the mass of one mole of something okay now a dozen is always tall the count the number is always tall if i take a dozen of two items two different items let's say for example a dozen donuts and a dozen eggs what can you say about their masses i didn't i don't want to know the value but i can say for sure the masses will be different is that right i can guarantee that i can take a package of dozen two packages i know for sure when i open the package it will have 12 of whatever it contains it doesn't matter but then if i want to know the masses of those two packages i know for sure i guarantee you the weight will be different mass will be different that's the same thing with the elements also a mole will have mole of any element will have this many atoms of that element but the masses will be different so what will be the mass of one mole of something so let's uh do uh some information here from the periodic table one mole by the way one more we write it in english like this but the unit for mole is mol do not confuse this with molecules there is also another unit we use molecules and do not confuse so mole is mol just like grams [Music] i would go but i decided to stay home and take my hair out okay that's what he looks like uh please remember that you are in a class and then this recording goes to several sections so it is very important that you mute your audio so one mole uh this is the unit for mole so one mole of let's say for example sodium sodium atoms right similarly take another one one mole of calcium will have this many number of calcium atoms when i put this number you know that right the significance i am trying to write the same number i don't want to write the number same again and again but what can you say about the masses like i said a package of two dozen package of two different things will have different mesh with elements what i can do is all i do is i can look at the atomic mass so sodium i look at here sodium has an atomic mass you know where the atomic mass is it's the number at the bottom of the periodic table so it is 22.99 grams this is the mass of one mole of sodium so when i go to the lab when i do want to do a reaction if i weigh 22.99 grams of sodium and put it in a reaction i know i am taking one mole and avogadro guarantees me it will have this many atoms okay so what is this 22.99 grams represent it is the mass of one mole of sodium is that right instead of seeing mass of one mole of sodium uh easiest way to say that is molar mass okay so we can simply say molar mass molar mass of sodium molar mass is nothing but mass of one mole of any substance that's what molar masses and people also use instead of molar mass they use the word formula mass which we learned from the previous chapter so that's pretty much the same okay molar mass or formula mass are pretty much the same so 22.99 grams if i take it i know that's a molar mass of sodium what about the molar mass of calcium when i look into my periodic table under calcium i have 40.08 grams so if i weigh 40.078 grams of calcium i know i have taken one more okay so mole is like i said it's a connecting bridge so how do we do problems with mole i'm going to like i said i'm going to draw a picture and it is pretty effective like in the sense when you you you can write this picture you can draw this picture on a little index card when you do work and also keep this if you happen to take chemistry later on so some other class some other you can have this i don't see this picture like anywhere there it's something like i was discussing with somebody many years back and then i just it came about and then from ever since i just draw it here okay for each class so this picture will basically solve all problems for mole concept so i'm going to have more in the center remember mole connects mass for our class there are so many connections going from moles but for this one this is mass and on the other side i have something called atoms it is atoms for elements it is molecules for compounds remember that okay but i'm not going to write molecules right now let me write it so that it can be registered once for all but for right now we are going to do atoms but molecules i have to look into the problems whether they have given any compound now supposing if i wanted to go from mole to mass i am going to draw this with an arrow here and with the number one i'm going to give you a formula for this from mass to mole i'm going to draw this yeah two and from all two atoms i'm going to have number three and from this from if i have something in my atoms if i wanted to convert this into mole i'm going to write four okay so this picture will be very handy why i'm saying this is you are going to see some similarities between the formulas here so formula number one is basically given mole time small or mass that's the formula if i have to go by route 1 that is if i have to use this route i am given something in mole if the question is asking what is the mass you have to go by this right so that's what it is so route number one has this formula given mole times molar mass route number two is given mass o divided by molar let me write down the formula then i will give you some quicker way of remembering them round number three given mole divided by avogadro now if you look at this one route 1 and 3 are multiplication route 2 and 4 are division is it right and whatever you use the term for route 1 you multiply it here you divide it here the same one by divided by molar mass here you are multiplying something by avogadro number you are dividing it by avogadro number you can see the similarity so the odd numbers are multiplication the even numbers are division if you can think like this i'm just trying to help you with the way you can you can remember these things okay now unfortunately i need the space so if you if you if you can write it down here i'm going to give you a few minutes just write it down on a piece of paper these four formula not necessarily this figure and what i will do is i will do some problems here okay so can you i know some of you might be taking this meeting over the phone you won't be able to do that but those who can do this just write them i need a few students when i ask the formula you should be able to tell me what it is have you finished taking it yes yeah i don't know it's your class or some other class told me like the other day like i was just standing there and then erase them they said hey i could give you some time so that we can see it on the camera so that's why all right is there a difference between a mole and avogadro's number i thought avogadro's number was a mole one mole like it is something like you are saying a dollar has hundred cents so it is like you can say a dollar one dollar or hundred cents right the same way i can say one mole a package i can say one mole or i can say avogadro number of atoms in that package okay so so on number three basically a given mole times avogadro's number you would basically you would be given a certain number of moles to multiply yes yes you got it you are right on the money okay so let us do a few problems here okay so here it is example 6.2 the example is worked out in the canvas i'm not going to do this so i'm going to do the problem the skill builder which is given right below it says calculate the number of grams of sulfur let me write it down this is 6.2 skill builder plus hopefully you can remember this this is 6.2 skill builder plus the question is asking you calculate the number of grams of sulfur calculate the number of grams of sulfur i'm writing this problem the rest of them i will just read it out for you calculate the number of moles of uh grams of sulfur in 2.78 moles of sulfur this is the problem and problems will be given to you like this okay uh somebody raised a hand what is it it says jasmine raised hand i don't know do you have a question if you have a question you can unmute and talk to me as long as you are you are in an atmosphere where there is no noise you can stay unmuted i don't have you don't have to mute it but unless you are in a parking lot listening to this on the phone some other noise that is when i want you to mute it okay okay coming back to this what information is given i am given something in more i am starting here that's my starting point what is the question is asking you what calculate the number of grams grams means it is asking you mess and here mass is here i so given some value i have to go from here to here how do i do this i have to use route number one right the formula for route number one and what is the formula for route number one given mole times molar mass okay so i take 2.78 given mole is 2.78 what is the molar mass of sulfur how do i do molar mass of sulfur i go to sulfur here and then find out sulfur is 32.065 grams so this is the atomic mass of sulfur supposing if you put a mole here you divide this by one more then it will cancel out okay the unit will cancel out so 2.78 2.78 times 32.065 giving me an answer of 89.141 grams of sulphur okay this is how you should show your what so this is an example 6.2 now i just gave you this one this will cover section of 6.1 to 6.4 all the way but they just split it up like a one section they say here 6.2 mole to mass the other one mass tomorrow will be another section but i have the figure here so i'm going to read uh i'm going to go over all these examples here okay the next one it is asking you is this example 6.3 skill builder i'm going to before i do this let's do skill builder 6.1 i i missed it here so example 6.1 and the skill builder below that i just did 6.2 6.1 skill builder thing one what it says is how many gold atoms are how many gold atoms how many gold atoms or in a pure gold ring in a gold ring they say pure gold ring because remember usually when you make ring and all they you cannot have pure gold it is you have to always mix it up with something else otherwise it will not really give you have you seen that if you it's a pure gold 24 karat gold or if you have something like a bangle or a ring you can just basically crumble up so usually what they do is they mix it with some other metals but uh something like copper and i don't know i i other elements are mixed here metals are here they say pure gold because they want you to think it is only gold nothing else a pure gold ring containing containing eight point 83 times 10 power minus 2 mole so this gold ring has the mole is 8.83 times 10 power minus 2 mole it is asking you how many gold atoms you have how should i do this what is given now in this problem i am given something in mole that means my starting point is right here what is being asked gold atoms i got to go here i'm here i gotta go there that means i have to use route number three what does the route number three says given mole times avogadro number right now this is exponent number also has exponent if you know how to deal with exponents it's great if not um if not what you can do is you can convert this into a decimal number it's a lot easier to handle it that way because you have to know when you multiply two exponents it will be the difference when you divide two numbers sorry when you multiply two numbers it is addition of exponents when you divide two numbers it is basically the difference i hope you understand what i'm saying 10 power 4 over 10 power 2 is basically the difference the answer is 10 power 2 if i have 10 power 4 times 10 power 3 the answer will be 10 power 7 you just add them if you have to multiply so if you know that rule it's fine but always what i will do is i will just convert this into a simple decimal number okay i have to take this picture so that i can work on more problems hopefully we will remember this okay remember mole is in the center mass is on the left side and atoms are on there and molecules are on the right side the top numbers are route odd numbers the bottom numbers are even numbers okay so i am given mole i have to find the atoms that means i have to use route number three the route number three basically says given mole times avogadro number instead of writing this number like this i'm going to write it simply like this 0.0883 this is the same number okay i'm just writing it in a decimal number so i take 0.08 and multiply by avogadro number when i do this what i will do is usually i multiply only the numbers i keep the exponent as it is if you know how to enter it in your calculator you can do so i'm not restricting you doing that but i my quickest way to do that is i will multiply these numbers keep the exponent if the number is not in scientific notation i will adjust them so let's see what we get here so point zero eight eight three times six point zero two two i'm getting this answer point you know this is not in scientific notation anybody why do you think it is not scientific notation you have to have one positive digit to the left of the decimal that means i have to move this guy one places when i move this guy one place is what should i do i have to reduce the exponent so i can simply say 5.32 times 10 power 22. so this many gold atoms will be present i'm just using formula three this is a little bit of math also involved in the sense like we are involving scientific notation how to do that but if you don't want to do this if you know how to multiply this number and this number on a calculator that's fine with me but you got to be careful there is a good big chance you may forget to put minus 2 okay so we have done 6.1 and 6.2 let's see 6.3 all of them based on that figure that's all it is okay example 6.3 from your book i'm going to do the skill builder it's asking you calculate the mass off 6.3 every problem i do i'm not doing the example i'm doing the skill builder problem right below that example it says calculate the mass of calculate the mass of 1.23 times n power 24 helium atoms so anybody can you tell me how to proceed with this what information is given i want one of you to answer this what information is given is it the mole the atoms atoms the number of atoms is given so where should you be on that figure i should be on the right most side is that right what is being asked in the problem calculate the mass i must come to the left most okay remember i have atoms and molecules on the right most side center mole and then mass here atoms are given that means i am right here on the right most side in that picture i have to get to the left most side there is no direct path what should i do i have to go from the left to the middle part using route number four you can look at the picture i go from this and then uh basically route number four will give me to the central bar from route number from the central part to the left i have to use route number one okay so in other words there are two steps here step number one in this is i'm going to go from atoms to molecules moles sorry in order to go from atoms to moles i have to use route number four that is formula number four what is the formula number four says given base unit divided by avogadro number so the given unit is this avogadro number is this if you can make a little adjustment to this number it is lot easier to handle this because if i get both of them in the same exponent i can simply cancel them this is 24 this is 23 how do i make it 23 i have to move the decimal place here so this is again you can simply enter that in the calculator i can you can do that i am just showing some shortcuts here i won't use a calculator at all i will simply write it like this if i move a decimal here it becomes 23 now i don't have to deal with this at all it cancels out okay so 12.3 over 6.022 is the easier number to enter on the calculator to me 6.022 basically i'm going to get 2.043 moles so the answer is 2.043 moles of what helium am i done with the problem the problem is asking you calculate the mass now from atoms i moved to moles so this is my answer 2.043 moles of helium i'm going to write it down here so that this is this many moles of helium i got this value here now i'm going to go from mole to mass okay so that's step number two step number two is step two is mole to mass how do i go from mole to mass using route number one what does the route number one says given mole times molar mass the given mole is this 2.043 molar mass of what helium so i go into helium it's right there on the rightmost side the molar mass of helium is 4.0026 grams so it is 4.0026 grams now what is this number this is the molar mass of helium okay so 2.043 times 4.0026 i'm getting an answer of 1.8 sorry 8.176 grams of helium so step number one i go from atoms to moles and then once i got into moles from moles i go to mass there are two steps involved here but still it is the same picture okay so if you can look at this one there are so many sections given here but pretty much it's the same idea now so far we dealt with only elements supposing if you are given some compounds how do i do this that's the next one again we are going to use everything the same formula except if i use element i can get the atomic mass straight from the periodic table if i am given a compound i have to find the molar mass using the formula mass which we learned from the previous chapter okay and that's what we are going to do here okay now i'm going to do skill builder 6.4 calculate the number of moles of no2 in 1.18 grams of inverter skill builder all right calculate the number of moles of n 2. calculate the number of moles of no2 in 1.18 grams of energy so what is given in the problem what information is given i see grams that means i have to start from mass that means i should be on the left most side what is being asked to calculate the number of moles so from left i got to get to the middle part i am on the left most side because mass is given and then i have to get to the middle part which is mole what is the formula what is the gravity we will use so i am going from grams to mole i have to use route number two what does the route number two says given mass over molar mass if it is nitrogen alone i can find it from the periodic table the molar mass because it is no2 i find the formula match that will be your molar mass so no2 means you have one nitrogen which is 14 point what's the number nitrogen is 14.007 and then this is 2 oxygen so 2 times 15.999 so it will be 14.007 plus 31.998 you add these two numbers this is the molar mass of no2 okay so if it is a simple element i can find it from the periodic table if it is a compound i have to calculate the molar mass using the formula mass which which we learned in chapter oh pi so what is the formula now i have to use step number route number two given mass over molar mass why i keep saying this formula over and over again because when we get two numbers we always have a tendency to take a bigger number divided by smaller number it doesn't have to be that's why i keep saying this basically it is given mass over molar mass so given mass is 1.18 grams of n divided by molar mass of no2 is 46.005 grams this is molar mass this is a given mass excuse me shock gets into after a couple of classes it's really too much dust to handle 1.18 over 46.005 the answer i am getting here is 0.0256 moles of enzyme now if i see this is the answer the problem is over supposing if i ask you how many molecules once you get to this you have to multiply this by avogadro my question is molecules this is moles so we are done right there okay when we um take the test can we use your little diagram with the little arrows no you can't use that okay know that all right because a diagram don't use it but i want you to know the formula that's why i gave you the shortcut like one one if i give you the formula in the test great but uh i i do not know sometimes in the test i may be tempted to give the formula sometimes i don't so that's why i don't want to commit but you have to know the formula but you can't have the figure why i'm doing this is if i let you use the figure everything like now in the final exam they say you can't do that then you are pretty much there are so many problems with the mole concepts you have to use more like mass to mole if you don't have that then you will be handicapped and then you will be oh i didn't realize that that's why i missed so many questions so it is better to learn the formula for these tests what else we have we have only this we have to find the formula mass that we will take use periodic table and this is the only one we are going to do moles so for this test you won't have too many um again i should not have mentioned the test here so basically for mole if you are going to deal only with mole concept you have to use only these formulas that's all okay one more ah example that will basically like finishes a mole okay so let's see what is this one here skill builder 6.5 how many water molecules are in a sample of water see now immediately they talk about molecules because it's a compound how many water molecules are in how many molecules of water are in a sample of water in a sample of water with a mass of 3.64 grams so the mass is given again this is example uh skin builder 6.5 sb plus skill builder plus okay because the example is given there and it's worked out for you okay now you tell me what is what is the information given there the mass mass is given that means i am on the left most side is that right yes what is being asked molecules how many milliliters be very careful it is not moles molecules that means i have to go to the right most side i'm starting from the left most side i got to go to the right most side that means from the left i need to go to the mole that is from mass to mole once i get the answer in mole i got to go from mole to molecules okay so step one pass tomorrow what i should use i have to use route number two given mass over molar mass molar mass of water is remember you have h2o each hydrogen is 1.008 times two hydrogen is one point zero zero eight since there are two hydrogen i put there plus oxygen is 15.999 so basically this is 2.016 uh plus 15.999 the answer i'm getting is 18.015 grams so this is the molar mass or formula mass of water 18.015 grams so when i go from mass to mole it is given mass which is 3.64 gram over molar mass which is 18.015 grams given mass over molar mass you can hear me saying this over and over again i don't go with the numbers twice so 3.64 divided by 18.015 the answer i'm getting is point zero let me write it down 0.2021 moles so when i do this step step number mass to mole is what step number two route number two i use the formula for two so i got this value okay so what i will do is the next one i am not done yet because i have to get the number of molecules so i have to use the next step now i got the answer in mole i have to convert them into molecules so i'm going to write the mole here so i won't lose that value 0.202 moles of water again do not confuse mole with molecules they will specifically say molecules don't treat this as mole okay in order to get molecules you have to get to the mole that is true for this problem so the next one is now that i have the more step number two now in the textbook if you do this everything is set in one step if you are comfortable with that i don't mind you don't have to show it like this but i just break them into smaller pieces so it's easier for you to see especially when you are doing this problem for starter so now i'm going from moles to molecules this is route number three what does the route number three says given mole times avogadro number so the given mole is this the avogadro number is this i will leave the exponent as it is divide multiply these two numbers so 0.2021 times 6.022 is 1.217 times 10 power 23 water molecules okay so that's basically how we do this if you look at this section now we covered section 6.1 to 6.5 but all depends on only small mole figure that's why i thought it is instead of me going through each section and then you will be worried what is math what is more more mole to math what are you saying i just put a figure just look at the figure just like a gps or more you look at the figure and then basically you should be able to go all right i hope it helps and this is basically concludes this concept mode and in the next class when i look at the other section i'm going to do them percent composition and then i will post the video for you but then i may do a one problem on percent composition whatever i i do it on the other sections okay so that way we get to cover more examples okay uh otherwise if i have to do everything for your class i can do lesser number of examples so it's easier for me to go that way i'm going to stop the recording here