acid + H2O → conjugate base + H3O+
If ONLY the acid is dissolved, the reaction to reach equilibrium is the only source of the conjugate base and the major source of H3O+.
The table below shows how, for this type of solution
The changes in concentrations of acid and conjugate base can be related to [H3O+].
[Conjugate base] and [acid] can be expressed in terms of initial concentrations and [H3O+].
| concentrations | H3O+ | conj base | acid |
| initial (before reaction) | 0 | 0 | c(acid) |
| change (due to reaction) | +[H3O+] | +[H3O+] | -[H3O+] |
| at equilibrium (after reaction) | [H3O+] | [H3O+] | c(acid) - [H3O+] ≈ c(acid)
if extent of reaction small |
The
extent of reaction depends on the strength of the acid and on its concentration.
If the acid is weak (
Ka < 10
–2) and
c(acid) > 0.01
, the extent of reaction is small.
As only a small amount of product forms, [acid] =
c(acid) - [H
3O
+] ≈ c(acid).
An alternative view uses mass balance:
c(acid) = [acid] + [conjugate base]
Where [conjugate base] << [acid], the approximation
c(acid) = [acid] is valid.
| Ka = | [conjugate base][H3O+] |
| [acid] |
| | reaction quotient |
Substituting the above for [acid] and [conjugate base] into the reaction quotient expression allows, for a solution of this type, calculation of
Ka from known [H3O+] or [H3O+] from known Ka .