AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), June 2025: Question 6

📐 Step-by-Step Calculation

1. Mass of gold in one phone:
   mass = moles × Aᵣ
   1.73 × 10⁻⁴ mol × 197 = 0.034081 g
2. Convert total phones to standard number:
   2.48 million = 2.48 × 10⁶ (or 2,480,000)
3. Total mass in grams:
   0.034081 g × 2.48 × 10⁶ = 84,520.88 g
4. Convert grams to kilograms (÷ 1000):
   84,520.88 ÷ 1000 = 84.52088 kg
5. Final rounded value:
   84.5 kg (or 85 kg to 2 s.f.)

🧠 Exam Technique & Alternative Method

You can also calculate the total moles first:

• Total moles = 1.73 × 10⁻⁴ × 2.48 × 10⁶ = 429.04 mol
• Total mass (g) = 429.04 × 197 = 84,520.88 g
• Convert to kg = 84,520.88 ÷ 1000 = 84.5 kg

❌ Common Errors & Traps

• Forgetting unit conversion: Leaving the answer as 84,520.88 g loses the final mark. Always re-read the required unit at the answer line ( kg ).
• Mishandling "million": Entering 2.48 × 10³ or 2.48 × 10⁵ instead of 2.48 × 10⁶.
• Calculator syntax errors: Forgetting brackets around powers of ten when multiplying standard form values.

✅ Correct Answer

Tick the 2nd box:

1.04 × 10²⁰

💡 Key Knowledge

Recall the relationship between particles and moles:

Number of atoms = moles × Avogadro constant (Nₐ)

= 1.73 × 10⁻⁴ mol × 6.02 × 10²³ mol⁻¹
= 1.04146 × 10²⁰ ≈ 1.04 × 10²⁰ atoms

❌ Why the Distractors are Wrong

• 2.87 × 10⁻²⁸ : Result of incorrectly dividing moles by Avogadro's constant.
• 3.48 × 10²⁷ : Incorrect power calculation (adding exponents without negative sign).

✅ Marking Points (3 Marks)

• Point 1: Uses bacteria [1 mark]
• Point 2: To produce a leachate (or solution containing gold / metal compounds) [1 mark]
• Point 3: Gold is then extracted/obtained by displacement or electrolysis [1 mark]

🧠 Exam Technique: The 3-Step Bioleaching Description

Whenever an exam asks you to describe bioleaching, structure your response chronologically:

1. The agent: State that bacteria are used.
2. The intermediate: State they break down the material to form an acidic solution called a leachate containing metal ions.
3. The final product: State how the solid pure metal is extracted from the solution (displacement using scrap iron or electrolysis).

❌ Common Errors

• Confusing bioleaching with phytomining: Mentioning plants, roots, or burning to ash gets zero marks for bioleaching. Phytomining uses plants; bioleaching uses bacteria.
• Stopping at "leachate": Many students forget to explain how the metal is actually recovered at the end (electrolysis or displacement).

✅ Acceptable Answers (Any Two)

• High-grade ores are still available (traditional mining is still economically viable) [1 mark]
• Takes a long time (slow process because plants have to grow) [1 mark]
• Land is not available (competes with land needed for agriculture/food production) [1 mark]
• Very small yields produced / low quantity of copper extracted [1 mark]
• New technology / still in developmental or unproven commercial stages [1 mark]

💡 Key Knowledge: How Phytomining Works

Plants absorb metal compounds through their roots from low-grade ore. The plants are harvested and burned to produce ash containing metal compounds. The metal is then leached with sulfuric acid to make a solution, which undergoes displacement or electrolysis.

❌ Examiner Pitfalls

• Vague answers: Simply writing "it's expensive" or "it's bad" does not get credit without qualifying (e.g., low yields make it economically uncompetitive).
• Saying it damages the environment: Phytomining actually causes less environmental damage than traditional mining, so this is scientifically incorrect.