This question is about weak acids.
Table 1 shows the pH ranges of some acid-base indicators.
| Indicator | pH range |
|---|---|
| Methyl orange | 3.1 – 4.4 |
| Methyl red | 4.4 – 6.2 |
| Phenolphthalein | 8.3 – 10.0 |
Identify the indicator from Table 1 that is most suitable for use in a titration between benzoic acid and sodium hydroxide.
Give the expression for the acid dissociation constant (KaK_{\text{a}}Ka) for benzoic acid (C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH).
Calculate the pH of a 0.150 mol dm−30.150\text{ mol dm}^{-3}0.150 mol dm−3 benzoic acid solution. Give your answer to 2 decimal places. For benzoic acid, pKa=4.20\text{p}K_{\text{a}} = 4.20pKa=4.20.
For benzoic acid, Ka=6.31×10−5 mol dm−3K_{\text{a}} = 6.31 \times 10^{-5}\text{ mol dm}^{-3}Ka=6.31×10−5 mol dm−3.
15.0 cm315.0\text{ cm}^315.0 cm3 of 0.120 mol dm−30.120\text{ mol dm}^{-3}0.120 mol dm−3 sodium hydroxide solution are added to 25.0 cm325.0\text{ cm}^325.0 cm3 of 0.120 mol dm−30.120\text{ mol dm}^{-3}0.120 mol dm−3 benzoic acid solution to form a buffer solution.
Calculate the pH of the solution formed. Give your answer to 2 decimal places.
A student plans to titrate benzoic acid solution with a solution of ammonia. Explain why this titration could not be done successfully using an indicator.
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.