 Let's talk about stoichiometry. Stoichiometry is the area of chemistry that deals with the quantities of reactants and products that are in a chemical reaction. Stoichiometry is something that you do every day. You just don't realize that you do it. For instance, when you're making a cheese sandwich, typically you need two pieces of bread and a slice of cheese to do it. If you're building a bicycle, you need two tires, a frame, and a pair of handlebars. It's no different in chemistry. Stoichiometry of a reaction is based on the balanced chemical reaction. Once you have the chemical reaction defined and the equation balanced, you can determine how much product you can make from a given amount of reactions. When I worked in the chemical industry, we would often get orders for compounds that were to be produced in large quantities. And based on that order, we would determine how much raw material to order to fill that particular product. There were times when we would not have enough of a raw material. In other words, the raw material ran out first, kind of like if you had 10 pieces of bread and only four pieces of cheese. You can't make all the sandwiches you need. So that would be what we call a limiting reagent. It runs out. So we have to use that limiting reagent concept when we're doing chemical stoichiometry. What we had to do in industry was we had to decide whether we could order more raw material or make a smaller batch of material that would be consistent with the quantity of the limited raw material. Whenever you do anything in chemistry, the first thing you do is balance the equation. Balancing the equation here would simply require adjusting the coefficients so that we have the same number of atoms on each side of the equation. When we do that, to balance the equation, this equation tells us we have two moles of aluminum plus three moles of chlorine, giving two moles of aluminum chloride. These coefficients become what we call a ratio. There's two moles of aluminum, three moles of chlorine, and two moles of aluminum chloride. Based on these ratios, we can create what are called conversion factors, which are fractions that tell us how we can convert quantities of one thing into another. Based on a value, let's say we wanted to produce four moles of aluminum chloride. And to do that, we need to know how much aluminum and how much chlorine we need to order into our plant to do that. The first thing we have to do is to decide on a ratio. Well, in the balanced equation, we have two moles of aluminum. We have two moles of aluminum chloride. All you have to do to get the ratio of aluminum to aluminum chloride is to write this ratio of aluminum over the ratio of aluminum chloride. And the ratio is called a conversion factor. We do the same thing here for chlorine. There's three moles of chlorine for every two moles of aluminum chloride. Now, if you talk in detail about conversion factors, you will note that we could have actually created an additional conversion factor for each of these two. Because you could also express this as two moles of aluminum chloride over two moles of aluminum. Either one of these would be the ratio. But the one that we need right now relates the aluminum to the aluminum chloride. And I'm going to explain why that is needed. Our goal is to produce four moles of aluminum chloride. So let's say that we wanted four moles of aluminum chloride. To determine how much aluminum is needed, we can use what's called a T-chart. We take the quantity of aluminum chloride that we want. We multiply it by the ratio of the reactant that we are working with. In this case, we have two moles of aluminum over two moles of aluminum chloride. Now, I want you to notice that you have a fraction being multiplied by this value. We can essentially cancel our units in the fractions. Because this would be in the top part of the fraction. This is in the lower part. The moles of aluminum chloride cancel out. And that leaves us with 4 times 2 aluminum over 2. And that's going to simplify to 4 aluminum. So what we need for aluminum is four moles of aluminum. We're going to do the same thing with the chlorine. We still have a goal of four moles of aluminum chloride. We're going to use our T-chart. The ratio between aluminum chloride and chlorine is expressed in this conversion factor. Three moles of chlorine over two moles of aluminum chloride. Once again, your moles of aluminum chloride are going to cancel. And you're left with 4 times 3 moles of chlorine over 2. If you're good at mathematics, you know that this can simplify. And so you're left with then 6 moles of chlorine. When you look at this problem, sometimes people that are good with relationships can actually work through this process without using a T-chart. I don't recommend it, at least when you're beginning to learn stoichiometry. Stoichiometry can be tricky. It can be a challenge. But it's not difficult if you, once again, practice. Practice as much as you can with stoichiometry. And I think that you'll be successful.