State and explain the law of mass action.
The law of mass action (Guldberg and Waage) may be stated as:
The rate at which a substance reacts is directly proportional to its active mass (molar concentration) and the rate of chemical reaction is proportional to the product of active masses of the reacting substances.
By the term active mass, we mean molar concentration or number of moles per litre. The active mass of A is denoted as [A].
Consider a simple reaction A 
Product
According to the law of mass action,
Rate of reaction 
where [A] represents the active mass of A, k is proportionality constant known as velocity constant. The velocity constant is also called velocity co-efficient or rate constant or specific reaction rate.
According to the law of mass action,
Rate of the reaction 
For a general reaction,
According to the law of mass action,
Rate of the reaction = 
Hence the law of mass action may also be stated as the rate of the reaction is proportional to the product of the active masses (or molar concentration) of reactants with each active mass raised to the power equal to the number of molecules of the respective reactants participating in the reaction.
