 Let's do this problem, very similar to the last problem in the lecture notes, only referring to the periodic table, how many grams does a mole of each of these molecules weigh, and how many molecules are in a mole of each of these substances. So the first one is water. So in order to figure out the mass of a mole of water, you've got to figure out what the molecular weight of one molecule is. So in order to do that, you've got to look at the periodic table, and the periodic table I'm using shows me to have hydrogen as an atomic weight of 1.0079, and there's two hydrogens in the molecule. So we're going to have to multiply that by 2, and then add that to the atomic weight of oxygen, which is 15.9994. And when we do that, we get the molecular weight. So let's write another column over here, molecular weight, 18.01. And then let's do the same thing for carbon dioxide, so 12.011 plus 15.99. Ammonia is 14.0067 for nitrogen, plus 3 times 1.007. So this one's going to be 17.030. So now we know the molecular weight of each of them. Well, the mass of one mole is the same number but different units. So the number is going to be 18.01. The units are going to be grams. So we can say the molar, so let's do another column. So the molar mass is going to be 18.0 always 12 eggs. It doesn't have donuts. It's always 12 donuts. So a mole or something is always this number, 6.022 times 10 to the corner. OK? So hopefully that helps everybody out.