Equivalent mass: The number of parts by mass of a substance that combines with or displaces 1.008 parts by mass of hydrogen or 8.0 parts of oxygen or 35.5 parts of chlorine is called its equivalent mass (EM). On the other hand quantity of a substance in grams numerically equal to its equivalent mass is called its gram equivalent mass (GEM) or gram equivalent.
Number of GEM = Mass of the substance in grams/GEM of the substance
Expressions for equivalent mass (EM)
(i) EM of an element = Atomic mass/Valency
(ii) EM of an acid = Molecular mass/Basicity
(Basicity of acid is the number of replaceable hydrogen atoms in one molecule of the acid).
(iii) EM of a base = Molecular mass/Acidity
(Acidity of a base is the number of replaceable–groups in one molecule of the base).
(iv) EM of a salt = Formula mass /Total positive or negative charge
(v) EM of an oxidising agent
= Formula mass/Number of electrons gained per molecule or Total change in O.N.
Equivalent mass of common oxidising agent changes with the medium of the reaction.