# How to Calculate Molar Mass Practice Problems

in this lesson we're going to learn how to

### calculate

#### molar

## mass

for a chemical compound by looking at its formula now your teacher or textbook might use one of these other names instead of#### molar

## mass

but the good thing is these are all pretty much just different names for the same thing so what we're going to learn here is going to work no matter which of these things you're talking about so we'll do a bunch of### practice

#### problems

start out with some relatively straightforwardexamples you can really get a hang of it and then we'll look at some ones that are a little bit more challenging as we move on so by the end of this you're going to be a total pro at calculating

#### molar

## mass

here's our first example so2 so to### calculate

the#### molar

## mass

for this compound we need to figure out how many of each type of atom there are in the chemical formula okay so we have an s without anything after it so that means that we have one sulfur atom right when you have achemical symbol without anything after it it means you have just one of them then we have Oh 2 which means that we have two oxygens here so one sulfur atom two oxygen atoms in this formula now we need to figure out how much sulfur atoms and how much oxygen atoms weigh and we can do that from the periodic table so we look up sulfur and oxygen on the periodic table and this is what we find and these numbers here are how much these various atoms weigh so now we add these together keeping in mind

how many of each atom we have so we have one sulfur atom so I'll do 1 times how much sulfur weighs which is 32 point zero seven plus two because I have two oxygens two times how much oxygen weighs which is 16 point zero zero now you don't really have to do the one here cuz I only have one sulfur but I just like to do it because I think it sort of makes things makes things more consistent and a little easier so one times the weight of sulfur times two x plus two times the weight of oxygen

gives us 64 point zero seven and the unit's here are grams per mole now what this means is that one mole of so2 weighs sixty four point zero seven grams that's what this grams per mole so how much does a mole of so2 weigh sixty four point zero seven grams let's keep going okay c3h8 Oh as before we look at the chemical formula to figure out how many of each type of atom we have here so C three means we got three carbon atoms H eight means we got eight hydrogen atoms and Oh without

anything after it means that we just have one oxygen atom now we look up each of these atoms on the periodic table to find out how much they weigh carbon is twelve point zero one hydrogen is one point zero one and oxygen is sixteen point zero zero now we add up the weights of these atoms keeping in mind how many of each atoms we have so we have three carbon atoms so we're going to do three times how much carbon weighs twelve point zero one plus eight because we have eight hydrogen atoms

times one point zero one which is how much hydrogen weighs plus one because we only got one oxygen atom one times sixteen point zero zero multiply these and add them together and we get sixty point one one don't forget the unit's grams per mole and once again this grams per mole here means that one mole of c3h8 o weighs sixty point one one grams this is the amount of grams that arm all of this weighs now let's keep going and look at some examples that are a little bit more complex

because they have parentheses in them which can get a little bit tricky okay see a and then we have parentheses no.3 - so let's use this formula to figure out how many of each type of atom we have the first thing we have is ca without anything after so that means that we have one calcium atom now we have no.3 but it's in parentheses and there's a 2 after that so what that means is that we have two no.3 s here they are no.3 no.3 now each of these no.3 s has one nitrogen and three

oxygens but since we have two of them we end up getting two nitrogens total and six oxygens total 1 2 3 1 2 3 ok so that means that total in our formula here we have one calcium atom and then two nitrogen atoms and six total oxygen atoms because this no.3 here is in parentheses and we have two of them now we just do the same thing we've done before we know how many atoms of each type we have so we look each atom up on the periodic table and find out how much it weighs now we go through the

calculations like we've done before one calcium so we do one times 40 point zero eight plus we've got two nitrogen so we do 2 times 14 point 0 1 plus 6 times 16 point zero zero because we have six oxygens and we end up with a hundred and sixty-four point one zero grams per mole and I know this might sound repetitive but it's really important to keep in mind what this number means and it means that one mole of ca no.32 weighs 160 four point one zero grams that's how much a mole of

this weighs let's do one more example with these parentheses just so we can get the hang of how we do the multiplication here okay parentheses NH 4 3 P o 4 so how many of each type of atom do we have well this part of the formula tells us that we have 3 NH fours so here they are 1 2 3 now how many total nitrogen and hydrogen's do we have well each NH 4 has 1 nitrogen and 4 hydrogens okay but we have three of these NH 4s that's what this 3 is telling us here so that means that total

since we have three of these we have in total three nitrogen atoms and twelve hydrogen atoms okay so that's what we get from this part because we have three of the NH Force now let's keep going the formula we got a p with nothing after it which means that we have one phosphorus out and then we have an O with four after it so that means we have four oxygen atoms okay I'm just going to get rid of this math stuff up here so that it can be a little bit clearer okay and now we know how

many of each type of atom we have so we just pull these up on the periodic table nitrogen hydrogen phosphorus and oxygen and we do the math here and we do 3 times nitrogen 14.0 1 plus 12 times how much hydrogen weighs one point zero one plus one because we only got one phosphorous times how much phosphorus weighs 30.9 7 plus 4 times 16 point zero zero which is how much oxygen ways and we end up with one forty nine point one two what are the units grams per which once again means that one mole of

nh4 Threepio four ways one 49.1 two grants this is how much a mole of this weighs let's do one more formula for something that's a little bit tricky we don't see it a lot but it can be very confusing mg so4 and then we have this dot and seven h2o this is a formula for compound called a hydrate which means that we have an mg s o4 and then we have seven water molecules h2o molecules we've got seven of these kind of just hanging out with this mg s o4 so let's see how we're

going to

### calculate

the#### molar

## mass

of this whole hydrate okay we're just going to read through the formula like we have before figuring out how many atoms of each type we have so we've got an mg with nothing after it and that means that we have one magnesium atom we got a sulfur and s without anything after meaning that we got one sulfur atom and then we got an O with a four after it meaning that we have four oxygens okay now with this seven h2o notice that we're not using parentheseshere okay no parentheses here but it's still kind of the same idea we don't use parentheses because of the because of the hydrate because we got this this up dot here but the seven h2o means that there are seven h2o s just like if we had parentheses okay so we got seven h2o here here they are one two three four five six seven seven H two O's in each h2o has two hydrogen's and one oxygen but we got seven of them seven h2o zero so that means that in total because we have seven of

these we end up getting 14 hydrogen atoms because they're two for each in there seven and then there's one oxygen in each of the h2o so we have seven oxygens total so that's how we deal with the seven - apart things again let me just clean up some of this math here so we can just be left with the atoms that we have in the formula okay so now we have our atoms here we got magnesium we got sulphur we got oxygen we got hydrogen and we got oxygen now if you're observant you might

realize we got oxygen in two places so instead of doing it separately I could add these up and say I have 11 oxygens total why am i doing it separately I don't really know maybe it's just because I think it's a little bit clearer but you can do whatever you like still I'm gonna do the oxygens separately because I just have it laid out like this anyway the math for this is going to be 1 times how much magnesium weighs 24.31 because I got one magnesium plus one times how much

sulphur ways plus I got four oxygens right here so 4 times 16 point zero zero plus I got 14 hydrogen's 14 times one point zero one and finally we got these other oxygens that are in the hydrate part and the water part of this so we got seven times sixteen point zero zero and again you could do the four plus seven you could do the oxygens all together and do sixteen times eleven but anyway we do this math multiplication addition add it all up and we get two hundred and forty six point four

six what are the units grams per mole meaning once again that one mole of MGS oh four seven h2o weighs 240 6.46 grams this the