Chemical equations are written in terms of moles not masses, so to calculate the mass of product produced from a known mass of reactant, we must first convert to moles. When in doubt, go to moles. After converting to moles, we can compare the number of moles of product produced from the given number of moles of product. This type of comparison is known as the mole method. When the molar amounts of reactants and products exactly match the amounts that appear in the balanced chemical equation they are said to be stoichiometric. Stoichiometry is the study of how much of what reacts with how much of what to form how much of what.