(a) Explain in terms of ionic equilibria why the aqueous solution of a salt derived from a weak acid and a strong base (e.g. sodium phenolate) has a pH greater than 7.

(b) By considering the equilibrium constant (K_a) of a weak acid, and the effect on K_a when a salt of that acid is added to the solution, derive the Henderson-Hasselbalch equation for buffer solution pH.

(c) Explain why the Henderson-Hasselbalch equation sometimes features a term written as + log₁₀ g_A- where g_A- represents the activity coefficient of the common anion, and hence describe the effect of increasing the concentration of both the acid and salt in the buffer formulation on buffer performance.

(d) An aqueous buffer solution is formulated containing 0.010 mol dm^-3 sodium phenolate to which phenol is added. Given that the pK_a of phenol is 9.98, calculate the concentration of phenol required to produce a pH of exactly 9.65 (you may assume that g_A- = 1).