AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 6

✅ Correct Answers

• Flame emission spectroscopy (1 mark)
• Flame test (1 mark)

💡 Key Knowledge

Potassium ions produce a characteristic lilac flame in a Bunsen burner flame test.

Flame emission spectroscopy is an advanced instrumental method that can detect and identify metal ions in mixtures even at low concentrations by analyzing light emission line spectra.

❌ Common Errors

• Paper chromatography: Used to separate soluble mixtures (such as food dyes or inks), not identify individual simple metal ions.
• Litmus paper: Tests for acidity/alkalinity (pH), not potassium ions.
• Boiling point: Shows presence of impurities, but is not specific to potassium.

🧠 Exam Technique

Read the question command carefully: "Tick (✓) two boxes". If you tick more or fewer than two, you risk dropping easy recall marks.

✅ Correct Answer

The colour of the precipitate formed is white.

💡 Key Knowledge

The question tells you potash alum contains potassium, aluminium, and sulfate ions:

• Potassium salts are always soluble, so K⁺ does not precipitate.
• Aluminium ions (Al³⁺) react with sodium hydroxide (NaOH) to form an insoluble white precipitate of aluminium hydroxide:
  Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s)
• Extra recall: If excess NaOH is added, aluminium hydroxide redissolves to form a colourless solution (unlike Mg²⁺ and Ca²⁺).

🧠 Linking Coloured Precipitates

• Blue: Copper(II), Cu²⁺
• Green: Iron(II), Fe²⁺
• Brown: Iron(III), Fe³⁺
• White: Aluminium, Calcium, or Magnesium

✅ Correct Answer

Sulfate ions can be identified using dilute hydrochloric acid and barium chloride solution.

💡 Key Knowledge

The test for sulfate ions requires two reagents:

1. Dilute hydrochloric acid (HCl): Added first to remove any carbonate ions that might also produce a misleading white precipitate.
2. Barium chloride solution (BaCl₂): Forms an insoluble white precipitate of barium sulfate:
   Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

❌ Common Errors

• Silver nitrate solution: Used to test for halide ions (Cl⁻, Br⁻, I⁻), not sulfate ions.
• Limewater: Tests for carbon dioxide gas (CO₂).

📐 Step-by-Step Calculation

1. Convert volume from cm³ to dm³:
   There are 1000 cm³ in 1 dm³.
   Volume = 800 ÷ 1000 = 0.8 dm³
   (1 mark awarded for conversion)
2. State and apply the formula:
   Mass (g) = Concentration (g/dm³) × Volume (dm³)
   Mass = 258 × 0.8
   (1 mark for setting up the multiplication correctly)
3. Calculate the unrounded mass:
   Mass = 206.4 g
   (1 mark for accurate unrounded value)
4. Round to 3 significant figures:
   Look at the 4th digit (4, so round down):
   Mass = 206 g
   (1 mark for correct significant figures)

✅ Final Answer

206 g

Full marks (4/4) are awarded for writing 206 on the answer line.

❌ Common Calculation Traps

• Forgetting to convert units: Multiplying 258 × 800 = 206,400 g loses the conversion mark and can propagate errors.
• Ignoring the rounding instruction: Writing 206.4 loses the final significant figures mark. Always check the required precision at the end of the question!
• Dividing instead of multiplying: Rearranging incorrectly as 258 / 0.8 or 0.8 / 258 .

🧠 Alternative Valid Method

Convert concentration to g/cm³ first:

Concentration = 258 ÷ 1000 = 0.258 g/cm³ (1 mark)

Mass = 0.258 × 800 = 206.4 g (2 marks)

Mass to 3 s.f. = 206 g (1 mark)