all right chemical equations give us mole ratios a dr. Kelly reaction last lecture we learned how to balance chemical equations now we're going to look at what they tell us in terms of the context of the mole real quickly however let's review what the mole is a mole is a named number we have lots of named numbers classic example is one dozen is another way of talking about 12 things made famous by Abraham Lincoln we know that one score of something is 20 if you've done construction then you may be familiar with the concept that 144 is very gross oh why 144 is so gross I don't know but we're familiar with the concept of named numbers a mole is the same thing one mole of anything is 6.022 times 10 to the 23rd this number is so important that it's actually named twice in addition to being known as the mole it's also referred to as Avogadro's number after the man who discovered it why do we care about it though what's so important about it all right if you're a pea chemist and listening cover your ears all right just or if you're not gonna cover yours just deal all right what's so important about it Avogadro's number is that 6.022 times 10 to the 23rd protons or neutrons since they have roughly the same mass has a mass of 1 gram what this relationships means is that we can read the atomic mass off our periodic chore chart not in terms of amu put in terms of grams in other words here's what the magic the mole does if I have one atom of carbon I have to go to the periodic chart and say that it's mass is 12.01 amu but if I have 6.022 times 10 to the 23rd carbon atoms then I can say that I have 12.011 grams of carbon and since we give this 6.022 times 10 to the 23rd a name and we named it the mole then I can go to my periodic table and I can say if I have one mole of carbon then I have 12.011 grams of carbon but wait there's more this works for any element on the periodic table I'm sure you have a periodic table in front of you and you can tell me if you have one mole of silicon how much silicon do you have you have 28 point zero eight five five grams of silicon if you have one mole of gold then you have one hundred and ninety six point nine six six six grams of gold when you have one mole of something you can read it uh you can read the mass off the periodic table in terms of grams this is awesome this provides us with the conversion factor between number of atoms and grams we can extend the principle of the mole beyond just individual atoms we can use that concept to determine the mass of a mole of a compound for example if I have one mole of methane then what mass of methane do I have so if I have one mole of methane what mass of methane do I have ooh what's with this ghosting effect I've got going on here all right so if I have one mole of methane then what mass of methane do I have well if I have one mole of methane that means I have one mole of carbon atoms and according to my periodic table one mole of carbon atoms is 12.011 grams if I have one mole of methane I also have four moles of hydrogen and each mole of hydrogen has a mass of 1.008 grams so my mass of so my mass in one mole of methane would be one times 12 plus four times zero point would be four Tom point zero three two or a total of 16 point four three grams of methane in one mole of methane so the mole is a very powerful tool it allows us to convert from mass to a number of molecules to a number of formula units to a number of atoms it provides us with a very very powerful tool let's work a couple more examples here again this is an abbreviated form of a lecture that I have a much more extended format that you should have listened to already at this point all right what is the mass of 4.2 moles of sodium sulfate if I was gonna work with work this with my line I'd start off and I'd say I've got 4.2 moles of sodium sulfate whatever I'm going to do next I'm going to have to get rid of moles of sodium sulfate hmm do I know a relationship between number of moles and mass why yes yes I do I always know the molar mass of a substance and remember the molar mass molar mass is the mass of one mole of a substance again review ah all right there we go so we can figure out the molar mass of sodium sulfate one mole of sodium sulfate contains two moles of sodium according to the periodic table I'd go to sodium and I find the molar mass of sodium and on my periodic table I'm looking at it is twenty-two point nine nine zero grams per mole then I have one mole of sulfur and according to my periodic table its I find sulfur I look there and I see that sulfur is 32 point zero six five grams per mole then I go to oxygen and I look there I have four oxygens in one mole of sodium sulfate and each oxygen mole is 16 point zero zero grams so now it's some crunching and I get two times twenty two point nine nine zero equals forty five point nine eight zero one times 32 is thirty two point zero six five and four times 16 is 64 add those wonderful numbers together plus 64 but you still need me will used today there we go so one mole of sodium sulfate is equivalent to one hundred and forty two point zero four eight grams of sodium sulfate so now I have a conversion factor between mass and moles I know that one mole of sodium sulfate is equivalent to one hundred and forty two point zero four five grams of sodium sulfate chemistry's easy life is hard yes all right now I've got my units to where they cancel out and I've gotten rid of moles of sodium sulfate and I'm left with units of grams so now I want to multiply everything on top divide through by everything on the bottom and when I do that I get time is four point two my calculator gives me five hundred and ninety six point five eight nine but look at my sig figs here I have one two two sig figs so my answer should have one two two sig figs so I'm going to round that away so after I've route through accounting for sigfigs I have 600 grams or if I want you to be really correct 6.0 times 10 to the 2nd grams of sodium sulfate chemistry's easy life is hard yes all right let's see you do it but before you try to do it why don't you write down Sylvester write down Sylvester s why L ve STR write down Sylvester it occurred to me that I may have some newbies listening in to have who may not have had me before in lecture and may have no clue as to what I'm doing up here with this line thing let me explain it all it is is this line is a way of stringing together ratios what I've really written here is if I clear everything out of the way whoops I meant to clear everything out of the way and now I'm just making a mess what I've really written here is I've written two ratios I've said 4.2 moles of sodium sulfate times the ratio of 1 mole sodium sulfate to 142 point zero four or five grams sodium sulfate all this line method that I use is as a way of stringing together ratios and this horizontal line represents the division symbol and where you see those X's you get a multiplication symbol so you'd have a multiplication symbol right there and a division symbol right there and you can see that when we look at the things down here write multiplication symbol multiplication symbol division symbol division symbol just a quick note on my notation here's one for you to try on your own if you're going to get the full benefit of this you've got to pause tried to try to work this yourself before I show you how to work it please I am begging you take a swing at this before you you you look at the answer pause me now Fosnight I said pause me pretty please alright alright some of you pause me others of you can't go a minute without listening to me and I understand that who doesn't want to spend lots of quality time with me alright so let's work this example we're gonna start our line with zero point five six seven moles of calcium hydroxide our first step is going to be to get rid of moles of calcium hydroxide do I know a relationship between moles of calcium hydroxide and the mass of calcium hydroxide sure I do absolutely I do 100% I do you bet your bottom dollar I know the relationship they've been going steady for about three weeks they've only had one sorry wrong wrong relationship there we go to the periodic chart and we can calculate the mass of one mole of calcium hydroxide one mole of calcium hydroxide contains one mole of calcium and according to the periodic table one mole of calcium has a mass of forty point zero seven eight grams and one mole of calcium hydroxide has two moles of oxygen where does that - come from right that - is outside the parentheses so I have not two moles of oxygen and two moles of hydrogen and each mole of oxygen has a mass of 16 point zero zero grams and each mole of hydrogen has a mass of one point zero eight eight grams and I got those all from the periodic table so then I do my math and I get forty point zero seven eight grams I get 32 grams and I get one point zero zero eight grams woops sorry can't multiply by four - that I can't multiply by two today can I do to do to do to do to do to do to do to do I get two point zero one six add everything together and I get seventy four point zero nine seventy four point zero nine four grams of calcium hydroxide in one mole of calcium hydroxide so now I have my conversion factor between moles and grams so I come back up here to my problem and I keep the numbers with their units one mole of calcium hydroxide is equivalent to seventy four point zero nine four grams of calcium hydroxide multiplied everything on top divide through by everything on the bottom and zero point five six seven times seventy four point zero nine four and I get forty two point zero one one coming off my calculator however if we look at our sig figs we've got one two three sig figs here so one two three so my answer should be forty two point zero grams chemistry's easy life is hard yes life is really really hard if you're Daffy Duck write down Daffy Duck da FF why have you ever noticed how Warner Brothers and Disney both made the angry animal ducks makes you wonder what happened in California that so many people were scarred and high angered heck I can consider Ducks such angry issues oh wait a second I bet I know look at the record of the Oregon Ducks compared to the California colleges at the moment all right let's keep going all right let's do a couple more quick review things for the mole here and again this should be review for you let's take a look at this type of problem how many moles of calcium hydroxide and ah how many moles of calcium hydroxide in 3.23 times 10 to the negative 4th grams of calcium hydroxide once again what are we going to do we're going to start our line with what we've got in the problem and that's three point two three times 10 to the negative 4 grams of calcium hydroxide so we start along with that we have grams up here and remember whatever unit I have right here from chapter one I automatically place down here no thinking no ifs ands or buts I have to get rid of that unit otherwise I wouldn't be doing the problem so I've placed those down there now I need a relationship between grams and moles do I have one yeah I calculated one last problem if you look at your paper from the last problem you'll see that we calculated the molar mass of calcium hydroxide to be one mole of calcium hydroxide equals seventy four point zero nine four grams of calcium hydroxide so there's our relationship between moles and grams I have grams down here this time so I need to write the numbers with their units seventy four point zero nine four is equivalent to one mole of calcium hydroxide so I'm going to multiply everything on top divide through by everything on the bottom and I get three point two three it's times ten to the negative fourth divided by seventy four point zero nine four and I get my calculator gives me the calculator lye four point three five nine three times ten to the negative six is the lie I only have three sigfigs here one two three so I'm going to come over here one two three and that gives me an answer of four point three six times ten to the negative six moles of calcium hydroxide chemistry's easy life is hard yes now let's see you work one of these all right here's one for you to do how many moles of potassium carbonate in twelve point three grams of potassium carbonate and apparently I am taking lessons from the William Shatner school of public speaking because I couldn't have read that any more choppy if I tried but now I'm sounding like robot I hope you paused me by right pause me right now and they're working on this on your own so pause me try it on yourself then come back and take a look at what I've got for the answer all right hopefully you worked at yourself how many moles of potassium carbonate and twelve point three grams potassium carbonate start your line with the only number given to you in the problem twelve point three grams potassium carbonate I have grams as my starting unit so I know my first step is going to involve cancelling out grams do I know a conversion factor between moles and grams yes the periodic table gives me one I know that one mole of potassium carbonate contains two moles of potassium according to the periodic table each mole of potassium has a mass of thirty-nine point zero nine eight three grams it has one mole of carbon at 12.011 grams and three moles of oxygen at fifteen point sorry sixteen point zero zero grams that is 48 grams that is 12.011 grams and that is mmm-hmm my darkest please two times thirty nine point zero nine eight three is seventy eight point one nine six six grams add them all together and that gives us one hundred and thirty eight point two one grams per one mole so one hundred and thirty eight point two one grams of potassium carbonate per one mole potassium carbonate multiply everything on top divide through by everything on the bottom and that gives me a calculator ly zero point zero eight eight nine nine six five five check my sig figs I've got one two three four four sig figs here so I need four sig figs here one two three four round on the basis of the six and that makes that a ten that attend and that a nine so my final answer and correct scientific incorrect not scientific notation incorrect sig figs if I wanted to put it in scientific notation which actually is a very good idea chemistry's easy life is hard yes all right now we've finished the review let's move on to the new part where we're going to roll this concept of going from grams to moles with information given to us by a chemical reaction and we're going to be able to do something that's going to be a daily task for a lot of you in the workplace or if you go into the laboratory environment this is going to be a task and you're going to be doing over and over again so let's find a chemical reaction to talk about anybody know a good one alright so let's take a look at this reaction this is the reaction of iron with oxygen to yield iron 3 oxide this reaction can be read in terms of moles we can say 4 moles of iron reacts with 3 moles of oxygen to yield 2 moles of iron 3 oxide this chemical reaction believe it or not is one big unit conversion this chemical reaction rate relates every chemical species in it to every other chemical species according to this chemical equation 4 moles of iron is equivalent to 3 moles of oxygen according to this chemical equation 4 moles of iron is equivalent to 2 moles of iron 3 oxide according to this chemical equation 3 moles of oxygen is equivalent to 2 moles of iron 3 oxide a chemical reaction provides a pathway to convert moles of any substance in it to moles of any other substance in it this is a very powerful tool it lets us answer some very important and very easy questions for example how many moles of oxygen would I need to react completely with 12 moles whoops I do there with 12 moles of iron how many moles of oxygen would I need to react completely with 12 moles of iron well we treat this just like any other unit conversion we start our line with a number given to us in the problem 12 moles of iron so I have 12 moles of iron I don't want moles of iron so I'm gonna place moles of iron down here where they'll cancel out whenever you switch substances you have to use a chemical reaction I'm wanting to go from moles of oxygen I'm going to go from moles of iron to moles of oxygen therefore I have to use a chemical reaction if I look at my chemical reaction my chemical reaction tells me four moles of iron is equivalent to three moles of oxygen four moles of iron is equivalent to three moles of oxygen so my unit conversion would be four moles of iron equals three moles of oxygen so I keep my numbers with their units four moles of iron is equivalent to three moles of oxygen so I multiply everything on top and I divide through by everything on the bottom and that actually winds up giving me nine moles of oxygen chemistry's easy life is hard yes there's nothing that says I have to stick between reactants for example I could do this by saying clean things up a little bit here I could ask myself how many moles of iron three oxide could I produce with 24 moles of iron and excess oxygen all right first of all you have just heard the most beautiful raw of chemistry not realizing how many moles of iron 3 oxide could I produce with 24 moles of iron and excess oxygen who doesn't like things that go to excess no one the reason excess is the most wonderful world in all of chemistry is that means you get to ignore whatever comes after it so we have oxygen that means we just get to forget about it we don't have to worry about the oxygen we just have to focus on the other species so I read this question and I've been given one number 24 moles of iron so I'm gonna use that to start my line with 24 moles of iron according to this chemical reaction my relationship between moles of iron and my moles of iron 3 oxide is 4 moles of iron can produce 2 moles of iron 3 oxide so it's a 4 to 2 ratio so my conversion factor is right there for moles place units where they cancel out I have moles of iron right here so I'm going to place some moles of iron down there keep their numbers with their units and I'm going to produce 2 moles of iron 3 oxide for every 4 moles of iron I have so I multiply everything on top divide through by everything on the bottom and I wind up producing 12 moles I've iron three oxide chemistry's easy life is hard yes let's keep going let's see you work one real quickly just to make sure you're with me and let's do some clean up clean up on aisle two continue to do two you to do and everything disappears okay try this question if you want to make 4.8 moles of iron 3 oxide how many moles of oxygen would you need alright pause me and try it yourself real quickly alright I hope you paused me so we're starting to looking at our problem we've got 4.8 moles of iron 3 oxide so we know we're going to start our line by writing 4.8 moles of iron 3 oxide that's going to go in our start position we don't know what we're doing next but we know that the units that are right here automatically go down there I know they gave me a lightsaber to play with how good is life I'm wanting to go from moles of iron 3 oxide to moles of oxygen according to my chemical equation what's the relationship for every two moles of iron I want to produce I need three moles of oxygen so my relationship would be three moles of oxygen to two moles of iron three oxide so keep their numbers in front of their moles three moles oxygen multiply everything on top divide through everything on the bottom and that gives you 72 no 64 of it will you still love me see do-do-do-do-do oh 7.2 I can be taught haha so that gives you seven point two moles of oxygen that you would need to make four point eight moles of iron three oxide chemistry is easy life is hard yes all right same reaction but this time we're moving it into the laboratory setting instead of going from moles to moles we're going to bring it close to the laboratory by starting with grams how many moles of iron three oxide can you produce with 4.2 zero grams of iron and excess oxygen all right we're gonna treat these problems and I get currently I'm on a roll to see how many times I can say all right in a single solitary lecture anybody at home tape and count but at any rate we're going to treat these just like we did the problems from chapter one we're going to look for our problem in whatever number they give us in this problem that's what we're going to start with they give us 4.20 grams of iron so we're gonna start our line with four point two zero grams of iron I have no clue how to solve this problem however I know my first step is going to be getting rid of grams of iron what's the only thing I know about grams and chemistry the only thing I ever know about grams and chemistry is the molar mass if I look at my periodic chart I see that one mole of iron has a mass of where did it go of fifty five point eight four five grams so I have a relationship between grams and moles will that get me anywhere no clue but it's better than nothing maybe the professor will give me some partial credit there so 55.8 for five grams of iron is equivalent to one mole of iron so I've gone from grams of iron over here to moles of iron over here and I've used my molar mass as a conversion factor all right so now I'm done with my grams but I have moles of iron and that's not what the problems asking for the problem wants to know how many moles of iron three oxide I can produce so I know my next step is going to involve getting rid of moles of iron do I know a relationship between moles of iron and moles of iron three oxide yes I do my chemical reaction gives it to me my chemical reaction says that four moles of iron is equivalent to two moles of iron three oxide so four moles of iron equals two moles of iron three oxide and I got that simply by looking at my chemical equation so now I've cancelled out my moles of iron and I'm meant for moles of iron two moles of iron three oxide using my stoichiometric relationship from a chemical reaction and somebody in my own my face-to-face asked about this our xn is chemical shorthand for reaction our x end is chemical shorthand for reaction whenever you switch substances you have to use the coefficients from a chemical reaction whenever you switch species you must use reactions alright so multiply everything on top divide through by everything on the bottom and that gives us calculators please I've got four point two divided by fifty five point eight four five divided by four times two and that gives me the calculator lai of zero point zero three seven six zero four eight that's my calculator lie check my sig figs I've got one two three sig figs here by the way these are exact numbers when we're talking sig figs this molar relationship here is considered exact because we're counting atoms is the idea so that molar relationship right there is exact so our sig figs are really going to be determined by our starting mass so I've got one two three sig figs one two three sig figs one two three round on the basis of the zero and that gives me a fine our final answer Alec zero point zero three seven six moles of iron three oxide is how much I can produce chemistry's easy life is hard yes are you ready I know you're ready let's kick this party up a notch alright let's go full bore full on all the way grams to grams how many grams of oxygen would you need to react with 72 grams of iron alright deep breath you have no idea how to work the problem not a big deal neither do i what I do know is that if I simply follow the units I'll be okay I have 72 grams of iron to start off with I know I don't want grams of iron so I'm know my next steps going to involve getting rid of grams of iron what do I know about grams of iron the only thing I ever know about grams is the molar mass I know according to the periodic table that one mole of iron is equal to 55 point 8 4 5 grams of iron so 55.8 4 5 grams of iron is equivalent to 1 mole of iron so I've gone from grams of iron to moles of iron using my molar mass as a conversion factor grams of iron have canceled out I don't want moles of iron though that's not what the problems asking for so I know my next step is going to be to getting rid of those same moles of iron that I worked so hard to get to what do I know about moles of iron well I know their relationship to moles of oxygen how do I know the relationship between moles of iron and moles of oxygen the chemical equation tells me it tells me I need 4 moles of iron to react with 3 moles of oxygen 4 moles of iron 3 moles of oxygen that's what my chemical equation is telling me 4 moles iron equals 3 moles oxygen so I place the numbers next to their units for moles iron 3 moles of oxygen do I want moles of oxygen no that's not what I'm being asked about I'm being asked about grams of oxygen so I know I'm gonna have to get rid of my grant my moles of oxygen so just to catch up a gun from moles of our iron now two moles of oxygen and I've used this coefficients for my chemical reaction to do so and now I want to go from moles of oxygen to grams of oxygen how can I get the grams of oxygen using the periodic charges conversion factor I always know molar mass thanks to the periodic table one mole of molecular oxygen contains two moles of oxygen atoms according to the periodic table each mole of oxygen atoms has a mass of 16 grams so one mole of molecular oxygen contains a mass of 32 grams so one mole of molecular oxygen has a mass of 32 grams this is the most common mistake I see on an exam by the way is oxygen can be tricky oxygen in its diatomic form one mole of it contains two moles of oxygen atoms so you have to remember to double that 16 so I've used the molar mass to go from moles of oxygen to mass of oxygen and I've just been placing my map my units where they cancel out all right grams to grams moles to moles moles to moles and I'm in grams so hey I have grams of oxygen that's the unit on being asked for my question so I must be done so now I'm going to multiply everything on top and divide by everything on the bottom and when I do that I get 72 divided by 55 point 8 4 5 divided by 4 times 3 times 32 and I get a calculator lie of 30.9 4 2 7 8 8 is the lie if I look at my sig figs 1 2 3 so 1 2 3 1 2 3 round on the basis of the floor and I get a final answer of thirty point nine grams of oxygen is how much oxygen I'd need to react with 72 grams of iron did I let's take a look at another I just noticed something here I hope you didn't throw people I made a little bit of a mistake here right that should be iron two three there we go I apologize hope that didn't cause anybody any major issues there okay here's a problem I want you to try on your own how many grams of iron three oxide can you make with 42 grams of oxygen and excess iron and buy on your own I mean you're gonna pause me work the problem then come back and check and I'm really begging you to I mean you can't see me but I have holes in my jeans because I'm on the ground begging you as hard as possible please please pause me and work this on your own alright I'm hoping you've paused me and I said all right again into lecture what is that number fifty but at any rate and how many but any rates if I put in here wow I'm a catchphrase King we look at our problem and we see one number to start with 42 grams so I have 42 grams of oxygen I don't want grams of oxygen so I know my next step is going to be to place grams of oxygen where they'll cancel out the only thing I ever ever know about grams of oxygen is the molar mass I mean the only thing I ever know about grams is molar mass one mole of oxygen contains two moles of oxygen atoms according to my periodic chart each one has a mass of 16 grams so 32 grams of oxygen in one mole of oxygen so 32 grams equals one mole of oxygen so I've placed grams where they cancel out I now have moles I don't want moles of oxygen however so in my next step I'm going to place my moles of oxygen where they'll cancel out do I know a relationship between moles of oxygen and grams of iron 3 oxide well I don't know relationship between moles of oxygen and grams by no relationship between moles of oxygen and moles of iron 3 oxide I know that 3 moles of oxygen equals 2 moles of iron 3 oxide so 3 moles of oxygen equals 2 moles of iron 3 oxide so now I've gone 2 moles of iron 3 oxide do I want moles of iron 3 oxide though no I want grams so I'm going to place those units where they'll cancel out as well form that same diagonal pattern right whatever units here goes here whatever units here goes here whatever units here goes here do I know a relationship between grams of iron 3 oxide and moles of iron 3 oxide sure I do of course I do I always know the molar mass of a substance I know that one mole of iron 3 oxide contains 2 moles of iron and according to my periodic table each one of those moles of iron has a mass of 55.8 4 5 grams and 3 moles of oxygen at 16 point zero zero grams each that gives me 111 plus 32 basically 55.8 95 times 2 plus 3 times 16 is 159 point seven nine grams so one mole is 150 nine point seven nine grams all right so now I multiply everything on top I divide by everything on the bottom and I get one hundred thirty nine point eight one hundred and thirty nine point eight grams of iron three oxide chemistry's easy life is hard yes what we saw and what we worked here is a pattern you'll use over and over and over and over again in chemistry the last two problems shared a similar set up those set up and that set up went grams of a two moles of a then I got rid of moles of a and I switched two moles of B and then I got rid of moles of B whoops then I got rid of moles of B and went to grams of B my pattern was grams of a two moles of a using molar mass as a conversion factor then I went to moles of B using my reaction coefficients and then I went to grams of B using the molar mass again a couple important things about this pattern that will help you from making mistakes whenever you're doing molar mass you're always doing it relative to one mole that means that there should always be a one right there and when you're going the other direction using molar mass again there should always be one because you're always relating one mole of whatever is here to a given number of grams 1 mole of whatever's here to a given number of grams remember whatever is above and below a line has to equal each other remember above has to equal what's immediately below it the only place where you can have a number of molds other than one is right here that's the only place you can have other something other than one a lot of the time those numbers will be one but that's the only place in this line where you can have a number other than one for a number of moles is when you're switching substances alright let's take a look at another reaction altogether but I haven't had you write down anything in ages you've probably been busy tweeting that I'm falling down on the job so why don't you write down Tweety everybody write down Tweety Bird twee T Y oh I just realized how is Warner Brothers not suing the heck out of Twitter's for Tweety really I would there's gotta be lawyers working on that I made Google that I made Google of Twitter's being sued by Warner Brothers woo how many media country companies am i wor running across on that one but everybody's writing down Tweety alright a new folk chemical reaction finally some of you are saying this chemical reaction is more substance awareness than it than the other one did so let's take a second just to write our relationships here the fact that we have more species doesn't matter if two compounds or two substances or two species are in a chemical equation they're related to each other just like it doesn't matter what color your cousin's hair is sorry Game of Thrones if they're in your family they're related to each other so according to this chemical equation two moles of lead to sulfur is equal to three moles of oxygen and two moles of lead two sulfur is equivalent to two moles of lead two oxide and two moles of lead two sulfur is equivalent to two moles of sulfur dioxide but wait there's more this is like the amazing ginza three moles of oxygen is related to two moles of lead to oxide and three moles of oxygen is equivalent to two moles of sulfur dioxide and there's still yet more according to this chemical equation two moles of lead to oxide is equivalent to two moles of sulfur dioxide if they're in a chemical equation they're related to each other by those chemical coefficients isn't life wonderful all right now let's get to the part where you'll all been dying for let's get to something where we can work a problem so let's see how many grams of lead to oxide can you produce from 72 grams of oxygen and excess led to sulfide alright you're panicking it's a test you have no idea how to work it deep breath just take it one step at a time first they've only given us one number so we know that's where we're going to start our line 72 grams of oxygen I don't want grams of oxygen so I'm going to place grams of oxygen where they cancel out the only thing I ever know about grams is molar mass if I'd go to my periodic table I see that one mole of oxygen contains two moles of oxygen atoms at 16 grams each for 32 grams so one mole of oxygen contains 32 grams of oxygen whoo I should have told you this pause me pause me try this on your own try this on your own okay did you pause me try it on your own hope so alright now I continue 72 grams of oxygen 1 mol 32 I've gotten rid of grams of oxygen and I now have moles of oxygen I don't want moles of oxygen so I know my next step is going to involve getting rid of moles of oxygen the only way I can switch substances is to use a chemical equation to convert moles of one substance to moles of another substance according to my chemical reaction three moles of oxygen because I have a three right here is equivalent to two moles of lead two oxide I have a 3 in front of my oxygen so I write a 3 in front of my oxygen down there I have a 2 in front of my lead 2 oxide so I write a 2 down there because those two are equal to each other I've now crossed out my moles of oxygen I have moles of lead oxide that's not what I'm being asked for in the question though I'm being asked for grams of lead to oxide so I have one more step I have to do I have to get rid of moles of lead oxide and go to grams of lead oxide the periodic chart always tells me the relationship between moles and grams one mole of lead oxide contains one mole of lead according to my periodic chart one mole of lead is two hundred and seven point two grams double checking here yeah and then one mole of oxygen is 16 point zero zero grams so 0.23 to two so 223 grams there 223 point 2 grams of lead oxide in one mole of lead two oxide because that's my molar mass so I've cancelled out my moles of lead two oxide I have units of grams of lead two oxide so I must be done now all I need to do is multiply everything on top divide through by everything on the bottom so that's exactly what I'm going to do and I'm going to go 72 divided by 32 times 2/3 times 220 3.2 and that gives me my calculator tells me three hundred and thirty four point eight grams of lead oxide sig fig check one two three sig figs here so one two three sig figs round on the basis of that eight and I get a final answer yes Alex that's my final answer 335 grams of lead to oxide tada why don't you write down Bugs Bunny write down Bugs Bunny be UGS be you and NY Bugs Bunny okay I don't know about you but I'm bored with oxygen you know that just sounds wrong but I we've we've looked at nothing but reactions that have oxygen so here's one without oxygen just for us to try something different and let's answer the question how many grams of nitrogen would you need to produce 42 grams of ammonia pause me and try this yourself seriously pause me and try this yourself I paused me I hope you did alright so we've got one thing to start our line with we're going from a mass of ammonia to a mass of nitrogen so we're going from mass to mass that means we can take comfort because we know we're going to follow that pattern we talked about earlier we know that we're going to follow that grams of a moles of a moles of B two grams of B pattern so we've got grams of a right here we don't want grams of a so we're going to place them where they cancel out the only thing we ever know about grams is molar mass we always know the mass of one mole of a substance because that's what the periodic table gives us one mole of ammonia according to the periodic table contains one mole of nitrogen at a mass of fourteen point zero zero seven grams and three moles of hydrogen at one point zero zero eight grams for a total of 17 something here fourteen point zero zero seven eight plus three times one point zero zero eight gives me 17 point zero three two grams so one mole of ammonia is 17 point zero three two grams of ammonia that's a three so now I've gotten rid of my grams of ammonia and I've gone from grams of a two moles of a and I don't want moles of a so I know I'm going to have to switch substances I'm going to have to switch substances using a chemical equation I have units of moles here so I'm going to place units down here so they cancel out so I've placed my units of moles where they cancel out I look at my chemical equation and I see that for every two moles of ammonia I have I need 1 mole of nitro nitrogen see it's a one here that's understood so it's a 1 to 2 ratio so two moles of ammonia require one mole of nitrogen so those have canceled out I don't want moles of nitrogen however I want grams of nitrogen so I place my moles of nitrogen where they're going to cancel out had moles of nitrogen here so I place them down here automatically I always know the relationship between grams and moles I know that one mole of nitrogen nitrogen is a diatomic molecule right see the little two there sees a little two there so one mole of nitrogen contains two moles of nitrogen atoms at fourteen point zero seven eight grams each so that gives me 2 times 14 2 times fourteen point zero zero seven eight that gives me twenty eight point zero one five six twenty eight point zero one five six multiply everything on top divide through by everything on the bottom forty two divided by seventeen point zero three two divided by two times twenty eight gives you do gives you a calculator ly gives me a calculator lai of thirty-four point five four two four eight but if I look at my problem I've got one two three sig figs so one two three and that gives me a final answer of thirty-four point five grams of nitrogen chemistry's easy life is hard if you have any questions on these asked you're more than ready to do them on your own now but by all means if you have issues on these ask me I'm here for you I'm willing to help you on wanting to help you let's roll