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A Level H1 Chemistry Acids Bases Salts Quiz
Free A Level H1 Chemistry Acids Bases Salts quiz, Exam version, with questions, answers, and A Level-style practice for Singapore students.
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A-Level Chemistry H1 Quiz - Acids Bases Salts - Answer Key
Total Marks: 25
Section A: Short Answer Questions [8 marks]
1. What is meant by the term weak acid? Illustrate your answer with an equation. [2 marks]
Answer: Definition: A weak acid is one that only partially dissociates/ionizes in water [1] Equation: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) [1] (Accept any weak acid with correct reversible arrow and state symbols)
2. Identify the Period 3 element that forms a sparingly soluble amphoteric oxide. [1 mark]
Answer: Aluminium (or Al) [1]
3. Explain why high acidity reduces the effectiveness of enzymes in biological systems. [1 mark]
Answer: High acidity denatures the enzyme, changing the shape of the active site, so substrate cannot bind and the reaction is not catalyzed [1] (Accept: H⁺ ions disrupt bonds in tertiary structure)
4. State the meaning of the term buffer solution. [1 mark]
Answer: A solution that resists changes in pH when small amounts of acid or base are added [1]
5. Write the expression for the acid dissociation constant, Ka, for ethanoic acid. [1 mark]
Answer: Ka = [CH₃COO⁻][H⁺]/[CH₃COOH] [1]
6. Distinguish between the terms concentration and strength when applied to acids. [2 marks]
Answer: Concentration: The amount of acid dissolved in a given volume of solution [1] Strength: The extent to which an acid dissociates in water [1]
Section B: Structured Questions [17 marks]
7. Carbonic acid, H₂CO₃, is formed when carbon dioxide dissolves in rainwater.
(a) Write a balanced equation, including state symbols, for the dissociation of carbonic acid in water. [2 marks]
Answer: H₂CO₃(aq) ⇌ HCO₃⁻(aq) + H⁺(aq) [2] (1 mark for correct formula and products, 1 mark for reversible arrow and state symbols)
(b) Write an expression for the acid dissociation constant of carbonic acid. [1 mark]
Answer: Ka = [HCO₃⁻][H⁺]/[H₂CO₃] [1]
(c) Explain why rainwater containing dissolved CO₂ has a pH less than 7. [2 marks]
Answer: CO₂ reacts with water to form carbonic acid [1], which partially dissociates to release H⁺ ions, making the solution acidic [1]
8. A student performs a titration to determine the concentration of a benzoic acid solution.
(a) Write a balanced equation for the reaction between benzoic acid and sodium hydroxide. [2 marks]
Answer: C₆H₅COOH(aq) + NaOH(aq) → C₆H₅COONa(aq) + H₂O(l) [2] (1 mark for correct reactants and products, 1 mark for balancing and state symbols)
(b) Calculate the concentration of the benzoic acid solution. [3 marks]
Answer: Moles of NaOH = 0.100 × (22.5/1000) = 0.00225 mol [1] Moles of benzoic acid = 0.00225 mol (1:1 ratio) [1] Concentration of benzoic acid = 0.00225/(25.0/1000) = 0.0900 mol dm⁻³ [1]
9. Ammonia solution can act as a buffer when mixed with ammonium chloride.
(a) Explain how this buffer system resists changes in pH when a small amount of acid is added. [2 marks]
Answer: NH₃ (weak base) reacts with added H⁺ ions [1] to form NH₄⁺, removing the excess acid and preventing large pH change [1]
(b) Write an equation to show what happens when a small amount of sodium hydroxide is added to this buffer system. [1 mark]
Answer: NH₄⁺(aq) + OH⁻(aq) → NH₃(aq) + H₂O(l) [1]
10. A solution of hydrochloric acid has a pH of 2.3. [3 marks]
Answer: [H⁺] = 10⁻²·³ = 5.01 × 10⁻³ mol dm⁻³ [2] Concentration of HCl = 5.01 × 10⁻³ mol dm⁻³ (HCl is strong acid, fully dissociates) [1]
Marking Notes:
- Accept answers to 2-3 significant figures
- Deduct marks for missing units where required
- Award partial credit for correct method with arithmetic errors