The strength of an acid can be determined by considering the extent
to which it dissociates in a given solvent (i.e. to which side this
The equilibrium constant in water is given by:
Note that the H2O term has been incorporated
into Ka, as water (the solvent) is in vast excess, and
hence the value of its concentration will change negligibly.
Ka is known as the acidity
constant of the acid. Its value will only be approximate
if concentrations are used as shown above; strictly the activities
of the species involved should be used. However, using concentrations
will be fairly accurate provided the solution is dilute. Generally,
the numerical value of Ka will be small, and for this
reason, Ka is usually converted into pKa
by the following equation:
For example, for methanoic acid, pKa =
Thus, the smaller the value of pKa, the
stronger the acid.
Similarly, the strength of a base is determined by
using pKb. For the example of BOH, a base, in water,
Kb can be determined by the following formula:
Again, note that the H2O term has been
incorporated into Kb, as water is in excess. pKb
can be found from Kb in exactly the same manner as pKa
can be found from Ka.