Equivalent mass is equal to the molecular or atomic mass divided by the number of electrons involved in the reaction per molecule, atom or ion.
For example in the reaction,

2H2O + 2e– → H2 + 2OH–

Two electrons are needed to produce one molecule of hydrogen gas.
So, 2 faradays of electricity are needed to produce one mole of the gas.

Hence, Eq. mass of hydrogen = Moler mass/2 = 2.016/2 = 1.008

For the reaction,

Cu2++ 2e– → Cu

per mole of copper atoms, 2 faradays are needed.

So, the equivalent mass of copper is half of the atomic mass.

Eq. mass of copper = Atomic mass of copper/2 = 63.5/2 =  31.75

It should be emphasized at this stage that the equivalent mass of any species is not simply a property of the species, but depends upon the reaction in which it participates, i.e.,one chemical species can have more than one equivalent mass depending upon the reaction it participates.

The equivalent mass of a substance is the quantity of material deposited or dissolved by 1 F(= 96500 C) of electricity.

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