Ka = [H3O+] ×  [conjugate base]
[acid]
For the system at equilibrium, combining [H3O+], [conjugate base] and [acid] as shown equals Ka for the acid-conjugate base pair.

In solutions where the concentration of the base of the conjugate pair equals that of the acid
[H3O+] must equal Ka for a system at equilibrium.
Thus pH is equal to pKa.

Ka = [H3O+] ×  [conjugate base]
[acid]
In solutions where the concentration of the base of the conjugate pair is higher than that of the acid
[H3O+] is lower than Ka for a system at equilibrium.
Thus pH is higher than pKa (on the base side).

Ka = [H3O+] ×  [conjugate base]
[acid]
In solutions where the concentration of the acid of the conjugate pair is higher than that of the base
[H3O+] is higher than Ka at equilibrium.
Thus pH is lower than pKa (on the acid side).

Thus for a solution in which
only the acid of the pair is dissolved, the pH must be both lower than 7 and lower than pKa.
only the base of the pair is dissolved, the pH must be both higher than 7 and higher than pKa.
Dr Sheila Woodgate
© The University of Auckland 2002