Reaction stoichiometry allows us to determine the amount of substance that is consumed or produced by a reaction. The following video considers the first part of this: how much of a reactant is consumed in a chemical reaction. Product formation is discussed elsewhere.
We are now ready to pull everything we know about reaction stoichiometry together, and answer the question: Given some initial amount of reactants, what should be present after a chemical reaction goes to completion? The procedure for answering this question is often called the "limiting reagent problem." You'll see why this name is appropriate in the following video.