A buffer solution of pH=4.40\text{pH} = 4.40pH=4.40 is required for preserving a soft drink. This acidic buffer is prepared by dissolving solid sodium benzoate (C6H5COONa\text{C}_6\text{H}_5\text{COONa}C6H5COONa) in a solution of benzoic acid (C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH).
The equation shows the dissociation of benzoic acid:
C6H5COOH(aq)⇌C6H5COO−(aq)+H+(aq) \text{C}_6\text{H}_5\text{COOH(aq)} \rightleftharpoons \text{C}_6\text{H}_5\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)} C6H5COOH(aq)⇌C6H5COO−(aq)+H+(aq)Calculate the mass, in g, of sodium benzoate that must be added to 250 cm3 of 0.150 mol dm-3 benzoic acid to produce this buffer solution at 298 K.
For benzoic acid, pKa=4.19\text{p}K_a = 4.19pKa=4.19 at 298 K.
Assume that the volume of the solution remains constant and that the sodium benzoate fully dissociates.
77 exam-style questions on AQA A Level Chemistry Acids and bases (A-level only), covering Brønsted–Lowry acid–base equilibria in aqueous solution (A-level only), Definition and determination of pH (A-level only), The ionic product of water, $K_w$ (A-level only), Weak acids and bases $K_a$ for weak acids (A-level only), pH curves, titrations and indicators (A-level only), and Buffer action (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.