 All right. The final of the four basic conversions that you have to be able to do to do stoichiometry is stoichiometry. It is the main conversion that we do when we do stoichiometry problems. That is using the mole ratio. Mole ratio problems always start with a balanced equation like this one. In this equation, these coefficients that we see or in this case don't see in front of the substances represent a ratio of substances that you either need or can produce in that reaction. We can interpret this 2H2 as meaning two moles of hydrogen. There's nothing in front of the oxygen. We can interpret that as meaning one mole of oxygen. We've got a two in front of the water. We can interpret that as meaning two moles of water. These are ratios. It's a two to one to two ratio. In other words, for every two moles of hydrogen we have, we would need one mole of oxygen for a complete reaction. We can also go across the arrow with this ratio. For every two moles of hydrogen we have, we can produce two moles of water. For every one mole of oxygen we have, we can produce two moles of water. These are ratios. We can set these ratios up as equivalences to solve problems and really make predictions, which is what stoichiometry is all about. These questions look quite different. The question could read like this. Given 3.5 moles of oxygen, how many moles of water can be produced? Not the straight up simple, convert so many grams of water into moles of water, or convert so many moles of water into molecules of water. It's not that simple sounding type of question. This one actually makes a prediction. This one sounds like some real chemistry here. If I have this much oxygen, how much water can I make with that? It's a real prediction. This is real chemistry. Again, this conversion here, this math here, is the heart of stoichiometry. What we do to solve this problem is first identify our substances. We have two substances in this question, oxygen and water. We find those two substances in our balanced equation. We set up an equivalence based on the number of moles we see. For our oxygen there's nothing in front of it. Again, we interpret that as meaning one mole. We look to our water and there's a two in front of it. We interpret that as meaning two moles. We have an equivalence. One mole of oxygen will give us two moles of water. I'll put oxygen there. One mole of oxygen will give us two moles of water, each two moles. With that we can do a little dimensional analysis just like we do with the other conversions. We always set up our equations starting with what we were given. We were given 3.5 moles of oxygen times, and then the conversion factor or a fraction that will cancel out, not only the given unit this time, but the given substance as well. I always give an oxygen. See, I can't look at the moles and decide what to put on the bottom because if I go by that they're both in moles I wouldn't know what to do. I have to go by the oxygen. I have moles of oxygen. So I've got to put moles of oxygen on the bottom. I'll put my two moles of water on top. Again, not only does the mole cancel, but more importantly the substance cancels. That's what this type of conversion is all about. Taking us from one substance to another. I've got a one on the bottom of this that I can ignore. The two's on top. Whenever the number's on top we multiply by it. 3.5 times 2 is 7. The unit is what we have left, moles of water. I think we always want to go back and make sure that's what we're asked to find. How many moles of water, moles of water, we got our answer. We can be reasonably certain that what we did here was correct. Unlike the other conversions, there are times when you'll have numbers in the numerator and denominator of your conversion factor. Again, it's really important to remember, numbers on top are multiplied by. Numbers on the bottom are divided by. So just work your way through it. Take your given times what's on top and then divide it by what's on the bottom and you'll get your answer. You won't always have a one in this. For example, when I decided I was going to do hydrogen and water, we'd have a two and a two. If you're not math oriented enough to recognize that that's just a one to one ratio and reduce it, you would multiply by two then divide by two. So be careful with this one. It's not just a single math operation in a lot of these problems. You have to both multiply and divide a lot of times. Next thing we do is start taking these four individual things and putting them together into multi-step problems.