a. Calculate the pH of the following buffer solutions:

i. 0.0500 mol dm^–3 methanoic acid and 0.100 mol dm^–3 sodium methanoate. (Ka of methanoic acid = 1.60 × 10^–4 mol dm^–3)

ii. 0.0100 mol dm^–3 benzoic acid and 0.0400 mol dm^–3 sodium benzoate. (Ka of benzoic acid = 6.3 × 10^–5 mol dm^–3)

b. How many moles of sodium ethanoate must be added to 1.00 dm³ of 0.100 mol dm^–3 ethanoic acid to produce a buffer solution of pH 4.90? (Ka of ethanoic acid = 1.74 × 10^–5 mol dm^–3) Hint: first find the hydrogen ion concentration, then rearrange the equilibrium expression to make [(sodium) ethanoate] the subject of the expression