The law of mass action states that the rate at which a chemical reaction takes place at a given temperature is proportional to the product of the active masses of the reactants. The active mass of a reactant is taken to be its molar concentration. For example, for a reaction xA+yB → products the rate is given by R=k[A]x[B]y where k is the rate constant. The principle was introduced by C. M. Guldberg and P. Waage in 1863. It is strictly correct only for ideal gases. In real cases activities can be used. See also equilibrium constant.
xA+yB → products