AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Higher), 2018: Question 6

✅ Correct answers

• CaO
• CO₂

Marks: 1 for each correct product formula. Either order is fine.

💡 Key knowledge

• Group 2 carbonates decompose when heated.
• The metal part becomes the metal oxide.
• The carbonate part gives carbon dioxide.

Equation: CaCO₃ → CaO + CO₂

🧠 Exam technique

• Use correct symbols and formulas, not names.
• Make sure the oxygen count is correct: carbonate turns into oxide + CO₂.

❌ Common errors

• Writing CaCO₃ again instead of the products.
• Giving CaCO or CO instead of CO₂.
• Writing names only — the mark scheme wants formulas.

📐 Calculations: step-by-step

1. Work out the mass of the carbonate part, CO₃:
   12 + (3 × 16) = 60
2. Subtract this from the total Mᵣ:
   197 − 60 = 137
3. The Group 2 metal has relative atomic mass 137, so the metal is barium.

The mark scheme says an answer of 137 scores both calculation marks, then barium / Ba scores the final mark.

✅ Correct answers

• Relative atomic mass = 137
• Metal = barium (or Ba)

💡 Key knowledge

• A carbonate contains CO₃ , so its formula mass includes 1 carbon and 3 oxygens.
• For a Group 2 carbonate, the formula is MCO₃ .
• So: Ar of metal = Mr of carbonate − Mr of CO₃ .

❌ Common errors

• Using 12 + 16 instead of 12 + (3 × 16).
• Forgetting to subtract the carbonate mass from 197.
• Not naming the metal after finding 137.
• Saying Ba without working is allowed for the name mark, but you need working for the calculation marks.

📐 Calculations: step-by-step

1. Choose two clear points from the line. Example from the graph: (0, 0) and (2.0, 330) or similar points on the line.
2. Use the gradient formula:
   gradient = change in y ÷ change in x
3. Substitute values:
   gradient = 330 ÷ 2.0 = 165
4. Suitable answers from the mark scheme are in the range 160–174, with the expected answer about 167.
5. Unit:
   cm³/g

✅ Correct answers

• Gradient = 167 (acceptable range 160–174)
• Unit = cm³/g or cm³ g⁻¹

If no other mark is awarded, the inverse gradient Δx/Δy or 0.006 can earn 1 mark only.

🧠 Exam technique

• Use points far apart to reduce rounding error.
• Read the axes carefully: x-axis is mass of carbonate (g), y-axis is volume of gas (cm³).
• State the unit clearly: gradient means y divided by x, so the unit is cm³ per g.

❌ Common errors

• Using the wrong axis order: mass ÷ volume instead of volume ÷ mass.
• Forgetting the unit or writing just cm³.
• Using points that are too close together and getting an inaccurate gradient.
• Missing the idea that marks can still be awarded for a correct method even if the final number is slightly off.

💡 Key knowledge

• At room temperature, 1 mole of gas = 24 dm³.
• From the graph, you need the mass of W that gives 240 cm³ of gas, then scale up to 24 dm³.
• Remember: 24 dm³ = 24 000 cm³.

📐 Calculations: step-by-step

1. Read from the graph:
   When volume = 240 cm³, mass of carbonate ≈ 1.45 g.
2. Use proportional scaling:
   24 000 ÷ 240 = 100
3. Multiply the mass by 100:
   1.45 × 100 = 145
4. So the relative formula mass of carbonate W is about 145.

The mark scheme accepts answers in the range 140–150. It also allows a method using your gradient answer from 06.3.

✅ Correct answer

Relative formula mass (Mᵣ) = 145

Acceptable range: 140–150

🧠 Exam technique

• Convert units carefully: 24 dm³ = 24 000 cm³.
• Show the ratio clearly. Examiners reward a correct proportional method.
• If you use the gradient method, rearrange y = mx to x = y ÷ m .

❌ Common errors

• Using 24 instead of 24 000.
• Reading the graph inaccurately.
• Forgetting to scale from the graph reading to one mole of gas.
• Mixing up whether the graph gives gas volume per gram or grams per gas volume.

💡 What top responses did

• Gave precise formulas in 06.1: CaO and CO₂.
• Showed clear subtraction in 06.2 and identified barium.
• Used a sensible pair of points for the gradient in 06.3 and included units.
• Converted 24 dm³ to 24 000 cm³ in 06.4 and used proportional scaling correctly.

❌ Where students lost marks

• Wrong product formula in 06.1.
• Arithmetic errors in calculating CO₃ mass.
• Gradient given without unit, or with the axes reversed.
• Not converting dm³ to cm³ before scaling in 06.4.

🧠 Quick checklist for full marks

• Products? Oxide + CO₂
• Carbonate mass? 12 + 3×16 = 60
• Metal mass? 197 − 60 = 137
• Gradient? rise ÷ run and include units
• Scaling? 24 dm³ = 24 000 cm³