Benzoic acid (C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH) is commonly used as a preservative in soft drinks to inhibit microbial growth. In an aqueous solution of benzoic acid at 298 K, the following equilibrium is established:
C6H5COOH(aq)⇌H+(aq)+C6H5COO−(aq) \text{C}_6\text{H}_5\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{C}_6\text{H}_5\text{COO}^-\text{(aq)} C6H5COOH(aq)⇌H+(aq)+C6H5COO−(aq)At equilibrium, the concentration of undissociated benzoic acid is [C6H5COOH]=1.25×10−2 mol dm−3[\text{C}_6\text{H}_5\text{COOH}] = 1.25 \times 10^{-2}\text{ mol dm}^{-3}[C6H5COOH]=1.25×10−2 mol dm−3 and the concentration of benzoate ions is [C6H5COO−]=8.85×10−4 mol dm−3[\text{C}_6\text{H}_5\text{COO}^-] = 8.85 \times 10^{-4}\text{ mol dm}^{-3}[C6H5COO−]=8.85×10−4 mol dm−3.
What is the value of the acid dissociation constant, KaK_aKa, for benzoic acid in mol dm−3\text{mol dm}^{-3}mol dm−3 at this temperature?
7.08×10−27.08 \times 10^{-2}7.08×10−2
6.27×10−56.27 \times 10^{-5}6.27×10−5
5.665.665.66
1.60×1041.60 \times 10^{4}1.60×104
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.