Note that the concentration of H2O is omitted from the Ka expression because its
value is so large that it is unaffected by the dissociation reaction.* The magnitude
of Ka provides information about the relative strength of a weak acid, with a
smaller Ka corresponding to a weaker acid. The ammonium ion, for example,
with a Ka of 5.70 ´ 10–10, is a weaker acid than acetic acid.
Monoprotic weak acids, such as acetic acid, have only a single acidic proton
and a single acid dissociation constant. Some acids, such as phosphoric acid, can
donate more than one proton and are called polyprotic weak acids. Polyprotic acids
are described by a series of acid dissociation steps, each characterized by it own acid
dissociation constant. Phosphoric acid, for example, has three acid dissociation reactions
and acid dissociation constants.