For any system at equilibrium, [acid], [conjugate base] and [H3O+] combined as shown equal Ka for the acid-base conjugate pair .
For solutions prepared by dissolving the acid, the conjugate base, or a mixture of the two in water, the pH can be calculated if both Ka and the initial concentrations of the substance(s) dissolved are known.
A scheme summarizing the general approach is shown below.
How do I know the equation for the reaction to reach equilibrium?
The reaction to reach equilibrium will be a proton transfer having water and the species dissolved as reactants.
| Ka = | [conjugate base][H3O+] |
| [acid] |
Proton transfer reactions are fast, and equilibrium is achieved on mixing.
For solutions prepared by dissolving the acid, the conjugate base, or a mixture of the two in water, the pH can be calculated if both Ka and the initial concentrations of the substance(s) dissolved are known.
A scheme summarizing the general approach is shown below.
| Given initial concentrations acid and conjugate base |  | Deduce equation for reaction to reach equilibrium |
 | Deduce [acid] in terms of unknown [H3O+] and initial concentration of acid | Given Ka  | Find [H3O+] |
| Deduce [conjugate base] in terms of unknown [H3O+] and initial concentration of conjugate base |
How do I know the equation for the reaction to reach equilibrium?
The reaction to reach equilibrium will be a proton transfer having water and the species dissolved as reactants.
If only the acid is dissolved, the initial concentration of conjugate base is zero, and
acid + H2O conjugate base + H3O+
If only the base is dissolved, the initial concentration of the acid is zero, and
conjugate base + H2O acid + OH–
acid + H2O
conjugate base + H3O+If only the base is dissolved, the initial concentration of the acid is zero, and
conjugate base + H2O
acid + OH–