UNDERSTANDINGS

• R3.1.9—The pOH scale describes the [OH–] of a solution. pOH = –log10[OH–]; [OH–] = 10–pOH

  • Interconvert [H+], [OH–], pH and pOH values.

• R3.1.11—For a conjugate acid–base pair, the relationship   Ka × Kb = Kw can be derived from the expressions for Ka and Kb.

  • Solve problems involving these values.

• R3.1.12—The pH of a salt solution depends on the relative strengths of the parent acid and base.

  • Construct equations for the hydrolysis of ions in a salt, and predict the effect of each ion on the pH of the salt solution.

• R3.1.13—pH curves of different combinations of strong and weak monoprotic acids and bases have characteristic shapes and features.

  • Interpret the general shapes of pH curves for all four combinations of strong and weak acids and bases.

• R3.1.14—Acid–base indicators are weak acids, where the components of the conjugate acid–base pair have different colours. The pH of the end point of an indicator, where it changes colour, approximately corresponds to its pKa value.

  • Construct equilibria expressions to show why the colour of an indicator changes with pH.

• R3.1.15—An appropriate indicator for a titration has an end point range that coincides with the pH at the equivalence point.

  • Identify an appropriate indicator for a titration from the identity of the salt and the pH range of the indicator.

• R3.1.16—A buffer solution is one that resists change in pH on the addition of small amounts of acid or alkali.

  • Describe the composition of acidic and basic buffers and explain their actions.

• R3.1.17—The pH of a buffer solution depends on both: (1) the pKa or pKb of its acid or base and (2) the ratio of the concentration of acid or base to the concentration of the conjugate base or acid.

  • Solve problems involving the composition and pH of a buffer solution, using the equilibrium constant.

MAKING CONNECTIONS