Stoichiometry /ˌstɔɪkiˈɒmɪtri/ is the calculation of reactants and products in chemical reactions.The extent of reaction ξ can be regarded as a real (or hypothetical) product, one molecule of which produced each time the reaction event occurs. It is the extensive quantity describing the progress of a chemical reaction equal to the number of chemical transformations, as indicated by the reaction equation on a molecular scale, divided by the Avogadro constant (in essence, it is the amount of chemical transformations). The change in the extent of reaction is given by dξ = dnB/νB, where νB is the stoichiometric number of any reaction entity B (reactant or product) and nB is the corresponding amount. Stoichiometry /ˌstɔɪkiˈɒmɪtri/ is the calculation of reactants and products in chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations among quantities of reactants and products typically form a ratio of positive integers. This means that if the amounts of the separate reactants are known, then the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined, then the amount of the other reactants can also be calculated.