AQA A-Level Chemistry Paper 3, 2019: Question 5

✅ Correct Answer

Fe + H₂SO₄ → FeSO₄ + H₂

Accepted alternatives:

• Fe + 2H⁺ → Fe²⁺ + H₂
• Fe + 2H⁺ + SO₄²⁻ → Fe²⁺ + SO₄²⁻ + H₂
• Fe + H₂SO₄ → Fe²⁺ + SO₄²⁻ + H₂
• Fe + 2H⁺ + SO₄²⁻ → FeSO₄ + H₂

❌ Common Errors

• Forming iron(III) sulfate: Fe₂ (SO₄)₃ . The prompt explicitly specifies that "all of the iron in the wire forms Fe²⁺(aq)".
• Writing hydrogen gas as monatomic H instead of diatomic H₂ .

📐 Step-by-Step Calculation

1. Inspect the titres:
   Titre 1 = 22.90 cm³
   Titre 2 = 22.70 cm³
   Titre 3 = 22.60 cm³
2. Select concordant results: Concordant titres must be within 0.10 cm³ of each other.
   Only Titre 2 (22.70) and Titre 3 (22.60) are within 0.10 cm³ (difference = 0.10 cm³).
   Titre 1 (22.90) is 0.20 cm³ away from Titre 2 and must be excluded.
3. Calculate mean:
   (22.70 + 22.60) / 2 = 22.65 cm³

❌ Common Errors

• Averaging all three titres: (22.90 + 22.70 + 22.60) / 3 = 22.73 cm³ receives 0 marks. Non-concordant titres must always be rejected.
• Trunctating or rounding to 1 decimal place (e.g. writing 22.7 cm³). Always quote titres to 2 decimal places.

✅ Correct Answer

5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O

💡 Key Knowledge

• Reduction half-equation:
  MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
• Oxidation half-equation:
  Fe²⁺ → Fe³⁺ + e⁻ (× 5)
• Multiply oxidation by 5 so the electrons cancel out completely upon addition.

✅ Correct Answer

colourless (or pale green) to (hint of) pink

Also allowed: "to pale / hint of purple".

🧠 Exam Technique: "First Permanent"

Potassium manganate(VII) is self-indicating:

• In the conical flask, Fe²⁺ is very pale green (often appears virtually colourless in dilute solution).
• MnO₄⁻ added is immediately reduced to colourless Mn²⁺.
• At the end-point, the very first drop of excess MnO₄⁻ gives a persistent pale pink colour.

❌ Common Errors

• Writing "to purple": examiners strictly reject "purple" alone because a full purple colour indicates you have overshot the end-point significantly.
• Reversing the order (e.g. pink to colourless). Always write Initial Colour → Final Colour.

✅ Correct Answers

• Taking the 25.0 cm³ portions: pipette (or volumetric pipette / graduated pipette)
• Adding the potassium manganate(VII) solution: burette

❌ Common Errors

• Writing "measuring cylinder" for the 25.0 cm³ portion (not precise enough for quantitative titration analysis).
• Writing "dropping pipette" or "teat pipette" (these are uncalibrated and not allowed).

📐 Step-by-Step Calculation

1. Convert mass to consistent units (grams):
   Mass of wire = 680 mg = 0.680 g
2. Determine total absolute uncertainty:
   The method states: "A container was weighed and its mass was subtracted from the total mass..."
   This requires two balance readings.
   Total uncertainty = 2 × (±0.005 g) = ±0.010 g
3. Calculate percentage uncertainty:
   % uncertainty = (Total uncertainty / Measured mass) × 100
   % uncertainty = (0.010 / 0.680) × 100 = 1.47% (or 1.5%)

❌ Common Trap: Single Reading Fallacy

The most frequent error is using only one uncertainty value:

(0.005 / 0.680) × 100 = 0.74%

Whenever an experimental quantity is found by difference (mass of container + contents minus mass of empty container), two readings are taken, so you must multiply the balance uncertainty by 2.