One equivalent of a substance in a reaction is defined as the amount of substance which reacts or liberates 1 mol of electrons ( or H+ or OH - ions). In a reaction a substance always reacts with another substance in equivalent amounts. The equivalent mass of a substance in a reaction is defined as the molar mass divided by the number of equivalents in 1 mol of the substance. The concentration of a solution may be expressed in normality ( Sumbol ; N) which is equal to the amount of solute in equivalents  dissolved in 1 dm3 of the solution. It has the units of equivalent per dm 3 and is abbreviated by the symbol N.The number of equivalents of the substance is given as

Number of equivalents = Mass of solute/equivalent mass.

The equivalent mass of a solute ( and also the normality of the solution) has to be determined with reference to a reaction since the solute may react in different ways in  different reactions. for example, KMnO4 oxidizes a reducing agent in different manners depending upon the medium as described in the following:

Acidic medium : MnO4-  + 8H+  5e-  ....> Mn2+   +4H2O

Weakly acidic or neutral or weakly alkaline medium : MnO4-  +3e-   + 4H+ .....>MnO2 + 2H2O

Alkaline medium : MnO4- + e- .....>MnO4 2-

Hence, Equivalent mass of KMnO4 in acidic medium = Molar mass of KMno4/  5