It states that, " Whenever two substance reacts the no. of gm equivalent of one substance is equal to the no. of gram equiv. of another substance and also the no. of gm equiv. of product is equal to that of reactant."

Let us consider a reaction A+B ---------------> product

According to the law of chemical equivalance;

At equivalence point,

No. of gm equiv. of A = No. of gm equiv of B .....................(i)

From Normality,

N= No. of gm equiv. of solute/ vol. of solution 

==> No. of gm equiv = N*V

For A: no. of gm eqvt. of A = NA * VA .............................(ii)

For B: no of gm eqvt. of B = NB* VB..............................(iii)

NA*VA = NB*VB

This is the required normality equation. This normality equation can be modified to molarity equation by multiplying acidity or basicity or charge or change in O.N.