Topic 18: Acids and bases - 18.2 Calculations involving acids and bases

Nature of science:

Obtaining evidence for scientific theories - application of the equilibrium law allows strengths of acids and bases to be determined and related to their molecular structure. 1.9


Essential idea: The equilibrium law can be applied to acid-base reactions. Numerical problems can be simplified by making assumptions about the relative concentrations of the species involved. The use of logarithms is also significant here.

The expression for the dissociation constant of a weak acid (Ka) and a weak base (Kb).

For a conjugate acid base pair, Ka × Kb = Kw.

The relationship between Ka and pKa is (pKa = -log Ka), and between Kb and pKb is (pKb = -log Kb).

Applications and skills

Solution of problems involving [H+ (aq)], [OH-(aq)], pH, pOH, Ka, pKa, Kb and pKb.

Discussion of the relative strengths of acids and bases using values of Ka, pKa, Kb and pKb.


The value Kw depends on the temperature.

The calculation of pH in buffer solutions will only be assessed in options B.7 and D.4.

Only examples involving the transfer of one proton will be assessed.

Calculations of pH at temperatures other than 298 K can be assessed.

Students should state when approximations are used in equilibrium calculations.

The use of quadratic equations will not be assessed.