A Level H1 Chemistry Acids Bases Salts Quiz

A-Level Chemistry H1 Quiz - Acids Bases Salts - ANSWERS

Section A: Short Answer Questions [12 marks]

1. What is meant by the term weak acid? Illustrate your answer with an equation. [2]

Answer: A weak acid is one that only partially dissociates/ionizes in water [1] Example equation: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) [1]

Marking notes: Accept any suitable weak acid. Must show reversible arrow (⇌) and state symbols for full marks.

2. State the meaning of the term amphoteric oxide. [1]

Answer: An oxide that can react with both acids and bases [1]

Marking notes: Accept "shows both acidic and basic properties"

3. Identify the Period 3 element that forms a sparingly soluble amphoteric oxide. [1]

Answer: Aluminium (or Al) [1]

4. Complete the following equation for the first dissociation of carbonic acid in water: [1]

Answer: H₂CO₃(aq) ⇌ HCO₃⁻(aq) + H⁺(aq) [1]

Marking notes: Must show correct charges and state symbols

5. Write an expression for the acid dissociation constant (Ka) of carbonic acid. [1]

Answer: Ka = [HCO₃⁻][H⁺] / [H₂CO₃] [1]

Marking notes: Square brackets essential; water omitted from expression

6. Explain why high acidity reduces the effectiveness of enzymes in biological systems. [2]

Answer: High acidity (low pH) denatures the enzyme [1] This changes the shape of the active site, so substrate cannot bind effectively [1]

Marking notes: Accept "disrupts tertiary structure" or "breaks hydrogen/ionic bonds"

7. State two characteristic properties of a buffer solution. [2]

Answer: (i) Resists changes in pH when small amounts of acid or base are added [1] (ii) Maintains approximately constant pH [1]

Marking notes: Accept equivalent statements about pH stability

8. Name the type of bonding formed when BF₃ reacts with NH₃. [1]

Answer: Coordinate covalent bond (or dative bond) [1]

9. Explain why the triiodide ion (I₃⁻) is linear in shape. [1]

Answer: The central iodine has 2 bonding pairs and 3 lone pairs, giving linear geometry to minimize repulsion [1]

Marking notes: Accept VSEPR explanation

Section B: Structured Questions [18 marks]

10(a) Explain how this mixture acts as a buffer when a small amount of hydrochloric acid is added. Include a relevant equation in your answer. [3]

Answer: The ethanoate ions react with the added H⁺ ions [1] CH₃COO⁻(aq) + H⁺(aq) → CH₃COOH(aq) [1] This removes most of the added acid, preventing a large change in pH [1]

10(b) Calculate the pH of this buffer solution. [3]

Answer: Using Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]) [1] pKa = -log(1.8 × 10⁻⁵) = 4.74 pH = 4.74 + log(0.20/0.15) = 4.74 + 0.125 = 4.87 [2]

Marking notes: Accept pH = 4.9 if rounded appropriately

11(a) Calculate the volume of sodium hydroxide required to reach the equivalence point. [3]

Answer: Moles of benzoic acid = 0.12 × (25.0/1000) = 0.003 mol [1] Mole ratio 1:1, so moles of NaOH needed = 0.003 mol [1] Volume = 0.003/0.10 = 0.030 dm³ = 30.0 cm³ [1]

11(b) Explain why the pH at the equivalence point is greater than 7. [2]

Answer: The benzoate ion (conjugate base) undergoes hydrolysis [1] C₆H₅COO⁻ + H₂O ⇌ C₆H₅COOH + OH⁻, producing OH⁻ ions [1]

12(a) Write the equation for the first dissociation of phosphoric acid in water. [1]

Answer: H₃PO₄(aq) ⇌ H₂PO₄⁻(aq) + H⁺(aq) [1]

12(b) Explain why the second dissociation of phosphoric acid is weaker than the first dissociation. [2]

Answer: In the second dissociation, H⁺ must be removed from H₂PO₄⁻ which already carries a negative charge [1] The electrostatic attraction makes it harder to remove the positively charged proton [1]

Section C: Data Analysis [10 marks]

13(a) Calculate the number of moles of aspirin in one tablet. [2]

Answer: Mass = 325 mg = 0.325 g [1] Moles = 0.325/180 = 1.81 × 10⁻³ mol [1]

13(b) Calculate the total mass of aspirin consumed over this period. [2]

Answer: Daily mass = 4 × 325 mg = 1300 mg [1] Total mass = 1300 × 5 = 6500 mg = 6.5 g [1]

13(c) Calculate the maximum number of moles of salicylic acid that could be produced from one tablet. [2]

Answer: From the equation, mole ratio is 1:1 [1] Maximum moles of salicylic acid = 1.81 × 10⁻³ mol [1]

14(a) Identify the equivalence point on the curve and explain your choice. [2]

Answer: The equivalence point is at the steepest part of the curve (inflection point) [1] This is where all the weak acid has been neutralized by the strong base [1]

14(b) Suggest a suitable indicator for this titration and justify your choice. [2]

Answer: Phenolphthalein [1] Its transition range (8.2-10.0) matches the pH at the equivalence point for weak acid-strong base titrations [1]

Marking notes: Accept other suitable indicators with correct justification

Total: 40 marks