Three buffer solutions are prepared at 25 °C using benzoic acid (pKₐ = 4.19) and…
CHEMISTRY · Multi-part · A Level · Chemistry · Acids and bases · Acids and bases
Problem
Three buffer solutions are prepared at 25 °C using benzoic acid (pKₐ = 4.19) and sodium benzoate. Each buffer has a total benzoate species concentration of 0.20 M but differs in the benzoic-acid–to-benzoate ratio, as shown in the table.
Buffer A: [HBz] = 0.15 M [Bz⁻] = 0.05 M
Buffer B: [HBz] = 0.10 M [Bz⁻] = 0.10 M
Buffer C: [HBz] = 0.05 M [Bz⁻] = 0.15 M
(a) Calculate the initial pH of each buffer.
(b) A chemist plans to add 0.0020 mol of solid NaOH to 100.0 mL of each buffer. Determine which buffer will experience the smallest change in pH upon the addition. Support your choice with appropriate quantitative work.
(c) The chemist instead adds 0.0020 mol of HCl(aq) to 100.0 mL of each original buffer. Determine which buffer now exhibits the smallest pH change and justify quantitatively.