In real life it is rare for a chemical reaction to use up all the reactants in the formation of products. When reactants are not used up in equal amounts, we say that the reactants are not present in stoichiometric quantities. Problems of this type are referred to as limiting reactant problems. They can be written in a variety of ways, but almost all require a mathematical determination using the mole method to identify the limiting reactant, and ultimately the extent to which product(s) can be formed.