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)

• The expression for the dissociation constant of a weak acid (K_a) and a weak base (K_b).
• For a conjugate acid base pair, K_a x K_b = K_w
• The relationship between K_a and pK_a is (pKa = -log K_a) , and between K_b and pK_b is (pKb = -log K_b)

Applications and skills:

• Solution of problems involving [H⁺ (aq)], [OH^-(aq)], pH, pOH, K_a1 pK_a1 K_band pK_b.
• Discussion of the relative strengths of acids and bases using values of K_a1 pK_a1 K_band pK_b.

Guidance:

• The value K_w 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.

• Mathematics is a universal language. The mathematical nature of this topic helps chemists speaking different native languages to communicate more objectively.

Utilization:

• Syllabus and cross-curricular links:
• Topic 8.1—conjugate acid–base pairs
• Topic 8.3—the pH concept
• Topic 8.4—strong and weak acids and bases
• Options B.7 and D.4—buffers

Aims:

• Aim 6: The properties of strong and weak acids could be investigated experimentally.