Equilibria: past paper questions

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• Question 2May/June 2026, paper 41Structured11 marks

  Define $\text{pH}$ mathematically.

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• Question 3Oct/Nov 2025, paper 42Structured15 marks

  Define conjugate acid–base pair.

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• Question 1Oct/Nov 2024, paper 41Structured16 marks

  Disodium phosphate, $({\mathrm{N}\mathrm{a}}^{+}{)}_2({\mathrm{H}\mathrm{P}\mathrm{O}}_4^{2-})$, reacts with an acid to form monosodium phosphate, ${\mathrm{N}\mathrm{a}}^{+}({\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4^{-})$. Identify the ions that are a conjugate acid–base pair in this reaction, using the formulae of the species involved.

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• Question 2Oct/Nov 2023, paper 41Structured14 marks

  Define $K_w$ mathematically by completing the expression.

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• Question 2May/June 2026, paper 42Structured10 marksHas a diagram

  Group 2 carbonates decompose on heating to form the corresponding metal oxide and carbon dioxide. The mechanism for this decomposition involves the carbonate ion breaking down to form carbon dioxide and oxide ions.

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• Question 3May/June 2025, paper 41Structured13 marks

  Chromium(III) hydroxide, ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$, is only slightly soluble in water. The value of the solubility product, $K_{\mathrm{s}\mathrm{p}}$, of ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$ is $1.0\times {10}^{-33}$ at $298\mathrm{K}$. Complete the expression for $K_{\mathrm{s}\mathrm{p}}$ of ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$.

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• Question 3Oct/Nov 2024, paper 42Structured19 marks

  The pH of a saturated solution of calcium hydroxide is 12.35 at $298\text{ K}$. Show that the concentration of hydroxide ions in a saturated solution of calcium hydroxide is $0.0224\text{ mol}{\text{dm}}^{-3}$ at $298\text{ K}$.

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• Question 2Oct/Nov 2023, paper 42Structured11 marks

  Benzoic acid, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}$, is a weak acid. The $K_a$ of benzoic acid is $6.31\times {10}^{-5}\text{ mol}{\text{dm}}^{-3}$ at $298\text{ K}$.

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• Question 4May/June 2026, paper 42Structured6 marks

  State what is meant by partition coefficient, $K_{\text{pc}}$.

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• Question 1May/June 2025, paper 42Structured8 marks

  Calcium nitrate, $\mathrm{C}\mathrm{a}(\mathrm{N}{\mathrm{O}}_3{)}_2$, decomposes on heating. Write an equation for the decomposition of calcium nitrate.

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• Question 5May/June 2025, paper 42Structured12 marks

  Chloric(I) acid, $\mathrm{H}\mathrm{C}\mathrm{l}\mathrm{O}$, is a weak Brønsted–Lowry acid. When chloric(I) acid is added to aqueous sodium hydroxide, an acid–base reaction takes place, as shown.

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• Question 1May/June 2024, paper 41Structured12 marks

  Describe the trend in the solubility of the hydroxides of magnesium, calcium and strontium.

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• Question 5May/June 2023, paper 41Structured13 marks

  The exhaust systems of most modern gasoline-fuelled cars contain a catalytic converter with three metal catalysts. These metals act as heterogeneous catalysts. Name three metal catalysts used in catalytic converters.

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• Question 3May/June 2025, paper 43Structured13 marks

  Chromium(III) hydroxide, ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$, is only slightly soluble in water. The value of the solubility product, $K_{\mathrm{s}\mathrm{p}}$, of ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$ is $1.0\times {10}^{-33}$ at $298\mathrm{K}$. Complete the expression for $K_{\mathrm{s}\mathrm{p}}$ of ${\mathrm{C}\mathrm{r}(\mathrm{O}\mathrm{H})}_3$.

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• Question 6May/June 2024, paper 42Structured13 marks

  Aqueous solutions of methanoic acid, $\text{HCOOH}$, and propanoic acid, ${\text{CH}}_3{\text{CH}}_2\text{COOH}$, are mixed together. An equilibrium is set up between two conjugate acid–base pairs. Define conjugate acid–base pair.

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• Question 6May/June 2024, paper 43Structured15 marksHas a diagram

  State what is meant by partition coefficient, $K_{\mathrm{p}\mathrm{c}}$.

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