Concept of equivalent mass

It is defined as the mass of an element/compound/ion which combines or displaces 1 part of hydrogen or 8 parts of oxygen or 35.5 parts of chlorine by mass.It is not always possible to apply this classic definition to determine equivalent weights of chemical entities. It is so because, we can not conceive of reactions involving chemical entities with three named reference of hydrogen, oxygen and chlorine. Generally, we are limited to determination of equivalent weights of elements and few compounds by using this definition of equivalent weight. A more workable definition is given as :

Equivalent weight,E=

Molecular weight
Valence factor
=
MO
x

Clearly, determination of equivalent weight amounts to determining valence factor “x”. Here, we shall classify chemical entities and the techniques to determine “x”.

or,

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,

two electrons are needed to produce one molecule of hydrogen gas. So, 2 Faraday of electricity is needed to produce one mole of hydrogen gas.

Hence,

For the reaction,

per mole of copper atoms, 2 faradays are needed. So, the equivalent mass of copper is half of its atomic mass.

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 value for its 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.