Polyvalence and Normality

The relative acidity or basicity of an aqueous solution is determined by the relative concentrations
of acid and base equivalents. An acid equivalent is equal to one mole of H+ (or H 3 O+) ions; a base
equivalent is equal to one mole of OH− ions. Some acids and bases are polyvalent, that is, each mole
of the acid or base liberates more than one acid or base equivalent. For example, the diprotic acid
H 2 SO 4 undergoes the following dissociation in water:

One mole of H 2 SO 4 can thus produce 2 acid equivalents (2 moles of H+) if dissociation is complete.
The acidity or basicity of a solution depends upon the concentration of acidic or basic equivalents
that can be liberated. The quantity of acidic or basic capacity is directly indicated by the solution’s
normality. Since each mole of H 3 PO 4 can liberate 3 moles (equivalents) of H+, a 2 M H 3 PO 4 solution
would be 6 N (6 normal).

Another useful measurement is equivalent weight. For example, the gram molecular weight of
H 2 SO 4 is 98 g/mol. Since each mole liberates 2 acid equivalents, the gram equivalent weight of

H 2 SO 4 would be ; that is, the dissociation of 49 g of H 2 SO 4 would release one acid
equivalent. Common polyvalent acids include H 2 SO 4 , H 3 PO 4 , and H 2 CO 3.