The Equivalent Weight of an element is that part by weight of it which combines with or displaces from a compound 1.008 parts by weight of Hydrogen or 8 parts by weight of Oxygen or 35.5 parts by weight of Chlorine.
Mg + H2SO4 = MgSO4 + H2
Here, 12 parts by weight of Mg displace 1.008 parts by weight of H2 from dil. H2SO4. So, the equivalent weight of Mg is 12.
Note: Equivalent Weight of an element is a pure number, it has no unit.
Equivalent Weight of Acid:
It is the quantity by weight of an acid that supplies one mole of H+ ion.
Equivalent Weight of an Acid = Mol. wt. of the acid / Basicity of the acid
Basicity of the acid = number of replaceable hydrogen atoms present in a molecule of the acid.
Equivalent Weight of Base:
It is the quantity by weight of a base that reacts with one mole of H+ ion.
Equivalent Weight of a base = Mol. wt. of the base / Acidity of the base
The acidity of the base = number of replaceable OH– ions present in a molecule of the base.