 Many times when we're talking about yields of reactions, we talk about theoretical yield. And the theoretical yield is important because theoretical yield will tell you what quantity of material you should get if everything is perfect. We all know that in most cases, things are not perfect. And sometimes a better gauge on a reaction's yield is percent yield. And percent yield is the actual yield that you obtained from your reaction divided by its theoretical yield multiplied by 100. Percent yield then tells you how efficient you are in processing the reaction. In industry, percent yield is very important because low yields tend to cost money. They tend to waste materials, and they certainly or something that management and operators are not happy about. So let's talk about how we calculate a percent yield from a reaction. If we look at this reaction where we began with two moles of hydrogen and two moles of oxygen, and our yield in this reaction was 24 grams, that's what we actually got out of it. What we would do in order to determine our percent yield would be to determine our theoretical yield, divide that into 24 and multiply by 100. In order to determine theoretical yield, we have to know what the limiting reagent is. And by calculations we can determine that hydrogen would be our limiting reagent here. Since there's two moles of hydrogen, let's decide how much water we should form in our process. Two moles of hydrogen times two moles of water divided by per two moles of hydrogen is our ratio of water to hydrogen. Note that moles of hydrogen will cancel. Note that your twos can cancel. And note that if we start with two moles of hydrogen, we will get two moles of water, theoretically. Now, two moles of water, how do we compare two moles of water with 24 grams of water? That's the question. If you remember, you can use molar mass in order to convert moles to grams for water. The molar mass of water is equal to 18.02 grams. We can set this up as a ratio, and we can say that there are 18.02 grams of water in one mole. Now, we learned that the theoretical yield for this process is two moles. So if we multiply this fraction times two moles of water, our moles will cancel. And we're left with 36.04 grams of water. Once we have grams here, it's an easier comparison between grams and grams. So in order to get our percent yield, we're going to divide 24 grams by 36.04 grams. We're going to multiply that times 100. And what we're going to find out is that our percent yield was close to 75 percent. Now, in some processes, 75 percent would be considered a good yield while in other processes, it would be considered a bad yield. It depends on the complexity of the process and other factors as to whether that would be the case. Percent yields and theoretical yields are in some ways a capstone for your stoichiometry because they allow you to use many different facets of stoichiometry to get this information. I hope that you'll practice it. I hope that you'll master it and that you'll know how to do this in the future.