hey guys hope you are well so this lesson is all gonna be about this formula over here okay so we know by now that n stands for the number of moles uh V is going to now be volume okay volume of gas and that is measured in decimeters decimeters and then VM is called molar volume okay it's also measured measured in decimeters some textbooks say that it's measured in uh decimeters per mole uh but just check what your teacher says if they want you to use that as the units or that that part's not too important okay now what's important is that this formula is a Formula that we use when we are only dealing with gas only gas you know you get solid liquid gas we only use this formula with gas okay now I forgot to mention this value over here it's a constant value which is going to be 22 comma 4. Now your teacher is going to tell you that you can only use 22 comma 4 when you are at something called s t p now STP just stands for standard temperature and pressure okay you don't really have to worry too much about that now in grade 10 but just know that the temperature is zero degrees Celsius and the pressure is you could either say one atmosphere you could either say 101 comma three kilopascals or you could say 101 300 pascals whatever one your teacher is using these are all the same thing actually but they're just different ways of writing it using different units okay so what does VM actually means it means the following it means that at STP so that's when you are at zero degrees Celsius and I'm just going to use one atmosphere of pressure then what they have discovered what scientists have discovered is that all well let's just say one mole of any gas will have a volume of 22 comma four decimeters it doesn't matter what the gas is it's any gas okay so they told that what they discovered was that if you have 22 well if you have one mole of any gas then the volume will be 22 comma four okay now let's go actually use this formula and you'll see that it's pretty easy this question says that we are given 44.8 decimeters now decimeter Cube that's a symbol for volume so that's this one over here so let's just mark off that we've got that of hydrogen gas at STP remember that stands for standard temperature and pressure determine the number of moles so we've got volume so we're definitely going to use this formula over here now remember that n stands for moles v stands for volume measured in decimeters and then VM stands for molar volume and that's always equal to 22 comma four decimeters okay it's like a magic number that you will be given in your test but you'll get used to it your your luck you'll memorize it anyways so that's that VM is this number over here so we can just go say that n is equal to 44.8 44.8 divided by 22 comma four and if you had to go work this out you end up with two Mo how easy is that now we're going to start for the next examples we're going to start adding in a few of the formulas that we've used in previous lessons such as this one and maybe even using something like that so we're going to learn to use all of them this question says determine the volume of 88 grams of CO2 okay so now they've given us grams so you can't use this formula immediately but we can use the other formula that we've learned which is this one over here where m is the mass in grams and then m is the mass on the periodic table it's called molar mass molar Mass also measured in well this one's actually measured in grams per mole and it is from the periodic table so what we could do guys is we could go use this formula to calculate the number of moles and then we could use this formula to calculate whatever they need okay so we're going to start with this formula so we could say n is equal to M over capital M and so that'll be equal to 88 over now to work out the mass of CO2 on the periodic table well there's one carbon so that is 12 and then there's two oxygens two oxygens so we'll say two multiplied by 16. and so if you had to go work that out you end up with 88 over 44 which is equal to 2 mole so we have two mole of CO2 now we're going to come back to this formula up here and we know the moles now and we also know VM remember VM is a constant of 22.4 so you've got to try think about how to get this by itself so you could make a triangle if you want like we've done in previous lessons the V is at the top the V is at the top and then you can just put the other two like that so if you want to get V bar itself then you would say these two are next to each other so you would multiply them and so you'd end up with two moles multiplied by VM which is 22.4 which is molar volume at STP and that would give you a value of 44 comma 8. Now the unit for volume is decimeters cubed this one says determine the volume when you've got this many now this is not moles this is the number of particles so here you would actually use this formula that we've learned about in previous lessons where n is the it's right to the Via n is the number of particles and then avogadros is a number that we've learned about which is 6.02 times 10 to the 23. so what we could do is we could work out the number of moles by saying n is the number of particles that they gave us which is this one over here okay divided by Avogadro's number and that'll give us 20. so we have 20 mole now we have the number of moles so we could come and use this formula so we could say n is equal to V over VM now we know the number of moles and we know this value but we have to try calculate this one so once again you could make a triangle which looks like this and then you could just get this by itself by saying V is equal to n multiplied by VM and so V is going to be equal to 20 moles multiplied by molar volume which is 22.4 and if you have to calculate this you end up with 448 decimeters cubed