Mandelic acid, C6H5CH(OH)COOH\text{C}_6\text{H}_5\text{CH(OH)COOH}C6H5CH(OH)COOH (pKa=3.41\text{p}K_a = 3.41pKa=3.41), is a weak monobasic alpha-hydroxy acid used in skincare formulations and as an antibacterial agent. A buffer solution is prepared by mixing 80.0 cm380.0\text{ cm}^380.0 cm3 of 0.400 mol dm−30.400\text{ mol dm}^{-3}0.400 mol dm−3 mandelic acid with 20.0 cm320.0\text{ cm}^320.0 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 this 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).
