I believe it is based on the law of conservation of mass; such that the total mass of reactants is equal to the total mass of the products.
Stoichiometry involves the study of the relative quantities of reactants and products in chemical reactions and how to calculate those particular quantities. The law of conservation of mass dictates that the quantity of each element must remain unchanged in a chemical reaction. Therefore; in a balanced equation each side of a chemical equation must have the same quantity of each element.
