 Now we're going to do some mole practice problems when we're talking about molecules. And so it's a little more complicated than with a single element, but all of the same thoughts and processes go into place. So you don't have to do anything too much more complicated, but it is a little more complicated, and so I wanted to do some practice problems with you. The molecular weight of a molecule, remember, is equal to the sum of all the atomic weights of the elements in the molecule. So before when we were doing single element problems, we just had to look up the atomic weight and we knew the grams per mole. In a molecule, we're going to have to add up all the atomic weights so that we know the grams per mole of a molecule. So if we're looking at one mole of calcium chloride, which is CaCl2, we have one atom of calcium and we have two atoms of chlorine. So we're going to go to our periodic table. We're going to see that calcium is 40.078 and chlorine is 35.453. So when we go back here, we'll know calcium 40.078 and chlorine is 35.453. Now, we only have one molecule of calcium, so we're fine to multiply this by one. But for this atomic weight, we have two molecules of chlorine, so we need to remember to multiply that by two. So we've got 40.078 and we've got 35.453 times 2, 70.906. And the molecular weight of the molecule is the sum of these. So 70.096 plus 40.078 is 110.984 grams per mole of calcium chloride. So when we need to know how much one mole of calcium chloride weighs, we can multiply by one mole and we get 110.984 grams. So let's do another example. This one's a little more complicated because we're not talking about just one mole. We're talking about 100 millimolar sodium chloride. There's one sodium and one chlorine. Chlorine we just looked at, it was 35.453. We'll look up our sodium, 22.990. Multiply both by one because there's one of each in each molecule. So we can add those together because we know multiplying by one isn't going to change them. So 22.990 plus 35, oops, 22.95.453, 58.443. So 58.443 grams per mole. But what we're looking for is 100 millimolar NACL. So again, we know that one mole is equal to 1000 millimole. So we can take this number and say when we're setting up our proportion, 58.443 grams per 1000 millimoles is equal to x per 100 millimoles. If we solve for x and multiply both sides by 100 millimoles, that leaves us with 10 on this side. So 58.443 divided by 10 is 5.844 grams per 100 millimoles. Again, if you don't feel comfortable with this kind of simplifying, you can see that 58.443 times 100 divided by 1000 is still going to give you the same answer. So you can feel comfortable doing it either way, whatever works the best for you. So that's some examples of how you might calculate the weight of a mole or a fraction of a mole or several moles of molecules.