 We're going to talk a little bit about some mole practice problems, and first we're going to focus on single elements. Remember the mole is a measure of, it's a number, it's 6.023 times 10 to the 23rd. And so all of these problems that we're going to do are talking about the weight, so we're going to determine the weight of one mole of magnesium. In order to do that we need to know the atomic weight of magnesium. So we go to our periodic table and find magnesium, and the atomic weight is 24.305, 24.305. And that is grams per mole. So if we need to know the weight of one mole, we can tell that it's 24.305 grams contains one mole of magnesium. The fact that it contains one mole of magnesium means that 24.305 grams contains 6.023 times 10 to the 23rd atoms of magnesium. We're going to look at one mole of phosphorus. It's going to be the same way to determine it. So we're going to go and look on the periodic table for the atomic weight of phosphorus. It's 30.947 grams per mole, so one mole of phosphorus weighs 30.947 grams. A little bit more complicated if we're looking at more than one mole or less than one mole. So we'll look at our periodic table. We're looking for calcium, 40.078 grams per mole. So you might look at that and automatically know that you need to multiply by 3 moles in order to get to your weight, so 40.078 times 3, that's not right, it's 120. 1, 2, 3, 4 grams. Or you might not know automatically to multiply by 3, and so you might want to set up a proportion. You can do this either way, and if you always set up a proportion, then no matter how complicated the number of moles we're looking for is, you'll always know how to solve that. So it's grams in one mole equals how many grams in 3 moles, and then you can multiply both sides by 3 moles, we'll cancel that. Moles will cancel here and we'll come up with 120.234 grams. So the same as we did by just multiplying by 3, but that's a way to ensure that you are doing it properly and not just kind of looking at it and assuming that you know what to multiply by. So with single elements, it's really straightforward, you use the atomic weight, which gives you a weight of grams per mole, and you convert it to the number of moles you need and that gives you the weight that you're looking for, for 3 moles of calcium or 1 mole of phosphorus. So I just want to do one more example of a single element mole problem, and this one, we're looking for the weight of 250 millimoles of sulfur. So again, we'll go to our periodic table, we'll look at sulfur's atomic weight, 32.065. So 32.065 grams per mole. We know that one mole equals 1,000 millimolars. So when we set up our proportion to sulfur 250 millimoles, we'll just substitute 1,000 millimoles for one mole. So we'll have 32.065 grams per 1,000 millimoles is equal to the weight of 200 x out of 250 millimoles. So we'll solve this proportion by cross multiplying 32.065 grams times 250 millimoles equals x times 1,000 millimoles. Divide both sides by 1,000 millimoles, that cancels each other, these millimoles will cancel. And so if we multiply, actually, we can simplify 250 out of 1,000 to 1 out of 4. And so if you divide, I lost my calculator, let's see, if you divide 32.065 divided by 4, x equal to 8.016. Just want to show you that this simplification did have us come up with the same answer that we would come up with if we had just multiplied it out, so 32.065 times 250 divided by 1,000. And we do still get the same number. So if you don't want to simplify and risk simplifying incorrectly, you can just put it all into your calculator and make sure that you get it right. So that's an example of something a little more complicated than whole numbers where we have a different unit and a different number of moles of a single element.