 So you're familiar now with how to calculate quantities of reactants and products in chemical reactions, but so far we've been calculating exact quantities. That is, we've been assuming that we always have exactly the right ratio of reactants as dictated by the mole ratio in the chemical equation. But what happens if we don't have exactly the right ratio of reactants? For instance, say we were reacting hydrogen with oxygen to give water, blowing up a hydrogen balloon for instance. We know that two moles of hydrogen reacts with one mole of oxygen. But what if we happen to accidentally mix two moles of hydrogen with two moles of oxygen? What would be left at the end of the reaction? Well, you may be able to work this out, but let me give you another analogy. I'm going to use this nice simulation from FET at the University of Colorado which uses the analogy of making sandwiches. So here we have a reaction in which two slices of bread are combining with one slice of cheese to make one cheese sandwich. So the ratio of bread to cheese reactants is two to one. Here below you can see that we have, at the moment, exactly that ratio, six slices of bread and three of cheese. And we end up with exactly the number of sandwiches that we expect, three. But what if we muck up the ratio of reactants and put in five slices of cheese? Well, what happens is that the bread runs out before all the cheese has reacted, and we're left with excess cheese, leftovers. All right, let's try it the other way round. The expected ratio is two to one, but if I have more than twice as much bread, let's bump it up to seven slices, then the cheese runs out first, and we have excess bread. Chemical reactions work the same way. The reaction will proceed as long as all the reactants are present. However, if one of them runs out, the reaction stops, even if there's some of the others left over. The reactant that runs out first is called the limiting reactant, because it limits how far the reaction can go. In this example it was the cheese. The reactants that aren't completely used up are called the excess reactants. Here it was the bread. So what's left sitting in the reaction vessel, or the plate, at the end, are as much product as was able to be made, plus whatever unused excess reactants are left.