When chemical reactions are performed commercially, it is important to know the relationships between amounts of active species, e.g. how little reactant is required in a reaction to make a lot of product. The study of the relationship between the amount of reactants and the amount of product is called stoichiometry. The stoichiometric amount is the exact amount of substance required or produced as determined by a balanced chemical equation. The reactant that determines how much product is formed in a reaction is called the limiting reagent (or limiting reactant). The maximum amount of product that can be formed depends on the amount of the limiting reagent, that reagent depleted first in the reaction. If more of a reagent is required than available, this reagent is the limiting reagent. If less of a reagent is required than is available, then this reagent does not limit the reaction and is said to be in excess. This program simulates reactions between two compounds forming one or two products. Four classes of reactions can be studied: precipitation, combustion, redox, and acid-base. The mass of products theoretically possible to obtain are calculated based on the mass of each reactant available and the reaction stoichiometry. A screen showing the calculations, used to arrive at the displayed results, is available. Overall, limiting reagents and stoichiometric factors are essential in the quantitative analysis of chemical reactions.