A Level Chemistry H3 Acids Bases Salts Quiz

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A-Level Chemistry H3 Quiz - Acids Bases Salts

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Duration: 60 minutes
Total Marks: 40
Topic: Acids Bases Salts (H3 Chemistry, syllabus-first practice; no past-year paper evidence available)

Instructions:

• Answer all 20 questions.
• Section A: short structured items (1–8). Section B: calculations and explanations (9–14). Section C: extended application (15–20).
• Show all working where calculation is involved. Use appropriate chemical notation.
• This quiz is generated from syllabus context only and is not derived from past exam papers.

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Section A: Foundations (Questions 1–8)

1. Define a Brønsted–Lowry acid and a Brønsted–Lowry base. [2]

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2. Write the conjugate base of $HCO_3^-$. [1]

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3. State the colour change of methyl orange in going from pH 2 to pH 10. [1]

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4. Give the formula of a salt formed by neutralisation of nitric acid with potassium hydroxide. [1]

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5. Explain, in one sentence, why a strong acid has a higher electrical conductivity than a weak acid of the same concentration. [1]

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6. Write the equilibrium expression for the dissociation of ethanoic acid, $CH_3COOH$, in water. [1]

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7. Identify whether $NH_3$ acts as an acid or base in liquid $NH_3$ autoionisation. [1]

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8. State the pH range in which phenolphthalein changes colour. [1]

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Section B: Calculations and Explanations (Questions 9–14)

9. A solution of hydrochloric acid has concentration $0.050\ \text{mol dm}^{-3}$. Calculate its pH. [2]

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10. The $K_a$ of methanoic acid, $HCOOH$, is $1.8 \times 10^{-4}\ \text{mol dm}^{-3}$ at 298 K. Calculate the pH of a $0.10\ \text{mol dm}^{-3}$ solution of methanoic acid. [3]

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11. $25.0\ \text{cm}^3$ of $0.100\ \text{mol dm}^{-3}$ NaOH is titrated with $0.100\ \text{mol dm}^{-3}$ HCl. (a) Calculate the volume of HCl required for neutralisation. [2] (b) State the pH at the equivalence point. [1]

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12. Explain, using Le Chatelier’s principle, how the pH of a buffer containing $CH_3COOH$ and $CH_3COO^-$ changes when a small amount of $H^+$ is added. [3]

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13. A student prepares a salt by reacting excess zinc carbonate with sulfuric acid. Write the balanced equation and state the method to obtain pure dry zinc sulfate crystals. [3]

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14. Compare the enthalpy of neutralisation of a strong acid–strong base system with that of a weak acid–strong base system. Explain the difference. [3]

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Section C: Extended Application (Questions 15–20)

15. A diprotic acid $H_2A$ has $K_{a1} = 1.0 \times 10^{-3}$ and $K_{a2} = 1.0 \times 10^{-7}$. (a) Write the two dissociation steps. [2] (b) Calculate the pH of a $0.020\ \text{mol dm}^{-3}$ solution of $H_2A$, stating any assumption made. [4]

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16. The following titration curve was recorded for a weak monoprotic acid titrated with strong base.

Image pending generation: graph for Q16.

Using the graph, determine the $pK_a$ of the acid and explain how the curve shows it is weak. [3]

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17. Describe how you would prepare a $pH = 4.75$ buffer using ethanoic acid ($pK_a = 4.75$) and sodium ethanoate. Include the ratio of concentrations required. [3]

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18. A salt $X_2CO_3$ is soluble. Describe two tests to confirm the presence of carbonate ion and state the observations. [3]

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19. Explain the role of an indicator in a strong acid–strong base titration and why methyl orange is suitable but not suitable for weak acid–strong base titration. [3]

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20. A sample of impure calcium carbonate reacts with excess HCl. $2.50\ \text{g}$ of sample produces $448\ \text{cm}^3$ of $CO_2$ at room temperature and pressure (molar gas volume $24.0\ \text{dm}^3\ \text{mol}^{-1}$). (a) Calculate the percentage purity of the sample. [4] (b) State one source of error in using volume of gas to determine purity. [1]

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A-Level Chemistry H3 Quiz - Acids Bases Salts (Answer Key)

Total Marks: 40
Topic: Acids Bases Salts (syllabus-first; not exam-derived)

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Section A Answers (1–8)

1. [2 marks]

• Brønsted–Lowry acid: proton ($H^+$) donor. [1]
• Brønsted–Lowry base: proton ($H^+$) acceptor. [1]
  Teaching note: This definition extends beyond aqueous systems; e.g. $NH_3 + HCl \rightarrow NH_4^+ + Cl^-$, where $HCl$ donates $H^+$.

2. [1 mark]
$CO_3^{2-}$
Remove one $H^+$ from $HCO_3^-$.

3. [1 mark]
Red to yellow (methyl orange: red below pH 3.1, yellow above pH 4.4).

4. [1 mark]
$KNO_3$
Neutralisation: $HNO_3 + KOH \rightarrow KNO_3 + H_2O$.

5. [1 mark]
Strong acid fully dissociates giving more free ions per unit volume, increasing conductivity.

6. [1 mark]
$K_a = \frac{[CH_3COO^-][H^+]}{[CH_3COOH]}$

7. [1 mark]
Base (autoionisation: $2NH_3 \rightleftharpoons NH_4^+ + NH_2^-$; $NH_3$ accepts $H^+$ to form $NH_4^+$).

8. [1 mark]
pH 8.2–10.0 (colourless to pink).

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Section B Answers (9–14)

9. [2 marks]
$HCl$ is strong: $[H^+] = 0.050\ \text{mol dm}^{-3}$
$pH = -\log(0.050) = 1.30$ [2 for correct value, or 1 if only formula]

10. [3 marks]
$HCOOH \rightleftharpoons H^+ + HCOO^-$
$K_a = \frac{x^2}{0.10 - x} \approx \frac{x^2}{0.10}$ (x small)
$x = \sqrt{1.8\times10^{-4} \times 0.10} = \sqrt{1.8\times10^{-5}} = 4.24\times10^{-3}$
$pH = -\log(4.24\times10^{-3}) = 2.37$ [1 method, 1 calc, 1 pH]

11. [3 marks]
(a) $n_{NaOH} = 0.0250 \times 0.100 = 2.50\times10^{-3}\ \text{mol}$
$V_{HCl} = n/c = 2.50\times10^{-3} / 0.100 = 0.0250\ \text{dm}^3 = 25.0\ \text{cm}^3$ [2]
(b) pH = 7.0 [1] (strong–strong)

12. [3 marks]
Added $H^+$ reacts with $CH_3COO^-$: $CH_3COO^- + H^+ \rightarrow CH_3COOH$ [1]
Equilibrium shifts left, free $H^+$ consumed [1]
pH changes only slightly due to reservoir of base form [1]

13. [3 marks]
$ZnCO_3 + H_2SO_4 \rightarrow ZnSO_4 + CO_2 + H_2O$ [1]
Filter excess $ZnCO_3$ [1]
Evaporate filtrate and crystallise; dry crystals [1]

14. [3 marks]
Strong–strong: ~ $-57.3\ \text{kJ mol}^{-1}$ [1]
Weak–strong: less exothermic [1] because energy used to ionise weak acid [1]

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Section C Answers (15–20)

15. [6 marks]
(a) $H_2A \rightleftharpoons H^+ + HA^-$ [1]; $HA^- \rightleftharpoons H^+ + A^{2-}$ [1]
(b) First dissociation dominates: $K_{a1} = x^2 / (0.020 - x) \approx x^2/0.020$
$x = \sqrt{1.0\times10^{-3} \times 0.020} = \sqrt{2.0\times10^{-5}} = 4.47\times10^{-3}$
$pH = -\log(4.47\times10^{-3}) = 2.35$ [2 calc, 1 assumption: $x \ll 0.020$ and $K_{a2}$ negligible, 1 final]

16. [3 marks]
$pK_a$ = pH at half-equivalence = 4.75 (from graph at 12.5 cm³) [1]
Weak acid: initial pH ~3 not ~1, equivalence pH >7 [2]

17. [3 marks]
Use Henderson–Hasselbalch: $pH = pK_a + \log([A^-]/[HA])$
For pH = pKa, ratio = 1 [1]
Mix equal concentrations/volumes of $CH_3COOH$ and $CH_3COONa$ [2]

18. [3 marks]
Add dilute acid: effervescence of $CO_2$ [1,1]
Pass gas into limewater: turns milky [1]

19. [3 marks]
Indicator shows pH jump via colour change [1]
Methyl orange suitable for strong–strong (jump includes 3.1–4.4) [1]
Not suitable for weak–strong (equivalence ~8–9, outside range) [1]

20. [5 marks]
(a) $n_{CO_2} = 0.448 / 24.0 = 0.01867\ \text{mol}$ [1]
$M_{CaCO_3} = 100.1\ \text{g mol}^{-1}$; mass pure = $0.01867 \times 100.1 = 1.87\ \text{g}$ [1]
% purity = $1.87 / 2.50 \times 100 = 74.8\%$ [2]
(b) Gas leakage / not STP [1]

Common mistake: forgetting to convert $cm^3$ to $dm^3$.