How to Calculate Molar Mass Practice Problems

In this lesson we are going to learn how to

calculate

the

molar

mass

of 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 cool thing is that these are all more or less different names for the same thing, so what we're going to learn here will work regardless of which of these things you're talking about, so we'll do a bunch of

practice

problems

and start with some relatively simple examples that you can really understand and then we'll look at some that are a little more challenging as we go, so by the end of this you'll be a Total professional in calculating molar mass.

Here is our first example, so2. To

calculate

the molar mass of this compound we need to calculate how many atoms of each type there are in the chemical formula. Okay so we have an s with nothing after it which means we have a sulfur atom right when you have a chemical symbol with nothing after it means you only have one of them so we have Oh 2 which means we have two oxygens here so a sulfur atom two oxygen atoms in this formula now we need to calculate how many sulfur atoms and how much oxygen atoms weigh and we can do it from the periodic table so we look for sulfur and oxygen in the periodic table and this is what we find and these numbers here are how much these different atoms weigh, so now we add them together taking into account how. many of each atom that we have, so we have one sulfur atom, so I'm going to do 1 times how much sulfur weighs, which is 32 point zero seven plus two because I have two oxygens, twice how much oxygen weighs, which is 16 point zero zero.

I don't really have to do the one here because I only have one sulfur, but I like to do it because I think it makes things more consistent and a little bit easier, so one multiplies the weight of the sulfur by two x more. twice the weight of oxygen gives us 64 point zero seven and the units 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 grams per mole are. So how much does a mole of so2 weigh sixty-four point zero seven grams, let's continue well c3h8 Oh, as before, we look at the chemical formula to determine how many atoms of each type we have here, so C three means we have three carbon atoms H eight means we have eight hydrogen atoms and Oh, with nothing after it, means we only have one oxygen atom.

Now we look for each of these atoms on the periodic table to find out how much they weigh. Carbon is twelve point zero one, hydrogen is point zero one and oxygen is. sixteen point zero zero now we add the weights of these atoms taking into account how many atoms of each one we have, so we have three carbon atoms, so we are going to do three times how much carbon weighs twelve point zero one plus eight because we have eight atoms of hydrogen multiplied by one point zero one, which is what hydrogen weighs plus one, because we only have one oxygen atom one times sixteen point zero zero, multiply them and add them and we get sixty point one one, don't forget the grams of the unit . per mole and once again these grams per mole here means that one mole of c3h8 or weighs sixty point one one grams this is the number of grams that all this weighs together now let's continue and look at some examples that are a little more complex because they have parentheses , which can be a bit complicated.

Well, look at, and then we have parenthesis number 3, so let's use this formula to calculate how many of each type of atoms we have. The first thing we have is ca. with nothing after it, that means we have a calcium atom, now we have the number 3, but it's in parentheses and there's a 2 after that, so what that means is we have two number 3s, here they are the number 3, the number 3. now each of these number 3s has one nitrogen and three oxygens, but since we have two of them, we end up getting two total nitrogens and six total oxygens. 1 2 3 1 2 3 ok, that means the total in our formula here we have one calcium atom and then two nitrogen atoms and six oxygen atoms total because this number 3 here is in parentheses and we have two.

Now we simply do the same thing we did before we knew how many of each type of atoms we have. So we look up each atom on the periodic table and find out how much it weighs. Now we do the calculations as we did before. One calcium, so we do one times 40 point zero eight, plus we have 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 one hundred sixty-four point one zero grams per mole and I know this may seem repetitive but it's very important to keep in mind. what does this number mean and does it mean that one mole of ca no.32 weighs 160 four point one zero grams, that's what one mole of this weighs.

Let's do one more example with these parentheses so we can understand how we do the multiplication here okay parentheses NH 4 3 P or 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 four, so here they are 1 2 3 now, how many total nitrogen and hydrogen are there? We have okay, each NH 4 has 1 nitrogen and 4 hydrogens, okay, but we have three of these NH 4, that's what this 3 tells us here, which means that in 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 Force NH.

Now let's continue with the formula, we have a p with nothing after it, which means we have a phosphorus outside and then we have an O with four after it, so that means we have four oxygen atoms. Well, I'm going to get rid of this math stuff here to make it a little clearer. Now we know how many atoms of each type we have, so we just take them out. in the periodic table nitrogen, hydrogen, phosphorus and oxygen and we do the calculations 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 have one phosphorus multiplied by how much phosphorus weighs 30.9 7 plus 4 times 16 point zero zero, which is the amount of oxygen, and we end up with one forty-nine point one two, what are the units in grams, which again means that one mole of nh4 Threepio, four forms , one, 49.1 two, grant this is the amount? one mole of this weighs let's make one more formula for something that's a little complicated, we don't see it much but it can be very confusing mg so4 and then we have this point and seven h2o this is a formula for a compound called to hydrate, which means that we have one mg so4 and then we have seven water molecules, h2o molecules, we have seven of these just hanging out with this mg so4, so let's see how we're going to calculate the molar mass of All this hydrate okay, we're just going to read the formula like we did before to find out how many atoms of each type we have, so we have a mg with nothing after it and that means we have one magnesium atom. a sulfur and an s with nothing after it, which means we have a sulfur atom and then we have an O with a four after it, which means we have four oxygens.

Okay, now with these seven h2o, note that we're not using parentheses here. Well, no parentheses here. but it's still the same idea we don't use parentheses because of the hydrate because we have this dot here but the seven h2o means there are seven h2o s like we have parentheses okay so we have seven h2o here is one two three four five six seven seven H two O in each h2o it has two hydrogens and one oxygen but we have seven of them seven h2o zero, that means that in total because we have seven of these we end up getting 14 hydrogen atoms because there are two for each of the seven and then there is one oxygen in each of the h2o, so we have seven oxygens in total, this is how we deal with the seven; aside from things again, let me clean this up a little bit. math here, so we can stick with the atoms we have in the formula.

Okay, now we have our atoms here, we have magnesium, we have sulfur, we have oxygen, we have hydrogen and we have oxygen. Now, if you are observant, you will notice. We have oxygen in two places, so instead of doing it separately, I could add them together and say I have 11 oxygens in total. Why do I do it separately? I don't really know, maybe it's just because I think it's a little clearer, but it can be done. Do what you want, but I'm going to do the oxygens separately because I have it like this anyway, the math for this is going to be 1 times the amount of magnesium that weighs 24.31 because I have one magnesium plus one time the amount of sulfur. plus I have four oxygens here, so 4 times 16 dot zero zero plus I have 14 hydrogens 14 times one dot zero one and finally we have these other oxygens that are in the hydrate part and the water part of this, so we have 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 mathematical multiplication, we add everything together and we get two hundred forty-six point four six, what are the units grams per mole, which means again that one mole of MGS oh four seven h2o weighs 240 6.46 grams, the molar mass is how much one mole of this weighs, so it doesn't matter what kind of chemical formula you get, if It has parentheses, it has a pair.

If it has a point, if it is a hydrate, it will be completely set, you must know exactly how to calculate the molar mass