9701 · 7.2

Brønsted-Lowry theory of acids and bases — practice questions

Practice and worked examples for 9701 Brønsted-Lowry theory of acids and bases. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

In the equilibrium $CH_3COOH(aq) + H_2O(l) \rightleftharpoons CH_3COO^-(aq) + H_3O^+(aq)$ , identify the two conjugate acid-base pairs.

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To identify the pairs, see which species transforms into another by losing or gaining a proton.

Pair 1: The ethanoic acid molecule, $CH_3COOH$ , donates a proton to become the ethanoate ion, $CH_3COO^-$ . Therefore, $CH_3COOH$ is the acid and $CH_3COO^-$ is its conjugate base.
Pair 2: The water molecule, $H_2O$ , accepts a proton from ethanoic acid to become the hydronium ion, $H_3O^+$ . Therefore, $H_2O$ is the base and $H_3O^+$ is its conjugate acid.

Worked example 2

Calculate the pH of a $0.0500,mol,dm⁻³$ solution of nitric acid ( $HNO_3$ ). Give your answer to two decimal places.

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Identify the acid type: Nitric acid is a strong acid, so it dissociates completely in water. $HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)$
Determine $[H^+]$ : Because dissociation is complete, the concentration of hydrogen ions is equal to the initial concentration of the acid. $[H^+(aq)] = [HNO_3(aq)]_{initial} = 0.0500,mol,dm⁻³$
Apply the pH formula: $pH = -\log_{10}[H^+]$
Calculate the pH: $pH = -\log_{10}(0.0500) = 1.3010...$
State the final answer to the required precision: $pH = 1.30$ (to 2 decimal places)

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