 The concept of stoichiometry involves a number of different topics. Today we'd like to talk about the topic of limiting reagent. Limiting reagent is much like other forms of stoichiometry. It's based on the balanced equation. So what we'd like to consider is this equation where hydrogen plus oxygen give water. The balanced equation for this is that two moles of hydrogen plus one mole of oxygen will give you two moles of water. This establishes the ratio of the products and reactants. If we are given that we're starting with two moles of hydrogen and two moles of oxygen it's important to know which of these would run out first in the process and i.e. be your limiting reagent. So to do that we base it on the ratio between the hydrogen and oxygen in the balanced equation. There are several ways you can do this and I'd like to show you both ways. The first way is to take the quantity of hydrogen, two moles of hydrogen and we're going to multiply that by the ratio of oxygen to hydrogen. And there is one mole of oxygen for every two moles of hydrogen. Remember that when you have a fraction of this type that hydrogens will cancel your left with oxygen. And what this tells us is that if you have two moles of hydrogen you need one mole of oxygen to react with it. At this point it's a simple matter of comparing the quantity of oxygen that you need with the quantity of oxygen that you have. Since you only need one mole of oxygen and there are two moles present that means that oxygen will not be your limiting reagent. Let's look at this from the other perspective. Let's look at it from the oxygen perspective. Again two moles of oxygen is our starting material but we're going to change our ratio. Now we're going to say that we have two moles of hydrogen for every one mole of oxygen. When we do this we find that we would need four moles of hydrogen to react with two moles of oxygen. We do a comparison again and we find that since we need two moles of hydrogen and we have only two moles we we need four moles of hydrogen we have only two moles that hydrogen is our limiting reagent. So this quantity of hydrogen will limit the amount of water that we can form. It is interesting to note that when you have a limiting reagent the quantity of product water that we form can be calculated from that limiting reagent and it's a simple matter to do. We say that there's two moles of hydrogen that's our starting point that's our limiting reagent times two moles of water for every two moles of hydrogen. We're going to cancel here and remember we can also cancel these two's. That leaves us with two moles of water and so what we're saying is that if we have two moles of hydrogen we can only produce two moles of water. This value two moles of water is known as the theoretical yield. Theoretical yields in other words it tells you the maximum quantity of product that you can form from your limiting reagent. It's like if you bake cookies and the recipe calls for 36 cookies theoretically you should only be able to make 36 cookies. The concept of theoretical yields and limiting reagent sometimes is confusing the students but it's it is a skill that you can master if you work on it and you do lots of problems.