How does rate law differ from law of mass action?

Explanation

Law of mass action is a theoretical relation between the rate of a reaction and the active mass of the reactants, participated in that reaction. However, reaction rate is experimentally deduced relationship between the rate of reaction and the concentrations of the reactant. Thus, the rate expression derived on the basis of law of mass action may or may not agree with the experirnental reaction rate. For example, in the reaction `2N_2O_5(g)to4NO_2(g)+O_2(g)`, according to the law of mass action, rate= `k[N_2O_5]_2` , but according to the experirnental result, rate= `k[N_2O_5]`.