Benzoic acid, C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH (pKa=4.20\text{p}K_a = 4.20pKa=4.20), is a weak monobasic acid often used as a food preservative. A buffer solution is prepared by adding 80.0 cm380.0\text{ cm}^380.0 cm3 of 0.500 mol dm−30.500\text{ mol dm}^{-3}0.500 mol dm−3 benzoic acid to 40.0 cm340.0\text{ cm}^340.0 cm3 of 0.600 mol dm−30.600\text{ mol dm}^{-3}0.600 mol dm−3 potassium hydroxide, KOH\text{KOH}KOH.
Explain why a buffer solution is formed.
Calculate the pH of the buffer solution that has been prepared. Give your answer to 2 decimal places.
A small amount of aqueous methylamine, CH3NH2(aq)\text{CH}_3\text{NH}_2(\text{aq})CH3NH2(aq), is added to the buffer solution. Explain, in terms of equilibrium, how the buffer solution would respond to the added CH3NH2(aq)\text{CH}_3\text{NH}_2(\text{aq})CH3NH2(aq).