AQA A-Level Chemistry Paper 3, 2024: Question 9

Core skill

• Interpret ionic names and write the correct ionic formula using charges.
• Recognise common ions: ammonium NH₄⁺ and ethanedioate (oxalate) C₂O₄²⁻.
• Balance charges to form a neutral salt and use brackets correctly.

Exam focus

• AO1 recall + application: ion charges and formula writing.
• Avoid “adding up atoms” without checking charge balance.

✅ Correct answer (what you should write/select)

B   (NH₄)₂C₂O₄

This gains the single mark because it is the only option with correct charge balance and correct use of brackets.

💡 Key knowledge

• Ammonium ion: NH₄⁺
• Ethanedioate ion (oxalate): C₂O₄²⁻
• A neutral ionic compound must have total charge = 0.

📐 How to build the formula (step-by-step)

1. Write the ions with charges: NH₄⁺ and C₂O₄²⁻ .
2. Balance charges: need 2 × (+1) to balance one (−2). So use (NH₄)₂ with one C₂O₄ .
3. Write the formula with brackets for a polyatomic ion used more than once: (NH₄)₂C₂O₄ .

Even though this isn’t a “number calculation”, it’s still a charge-balancing calculation.

🧠 Exam technique (how to secure the mark fast)

• Translate the name → ions → charges before looking at the options.
• Spot the “2−” ion (C₂O₄²⁻) and immediately think: “I’ll need two 1+ ions.”
• Check for brackets when there is more than one ammonium ion.

❌ Common errors (why students lose the 1 mark)

• Forgetting ethanedioate is 2− and writing NH₄C₂O₄ (overall charge would be +1 − 2 = −1, not neutral).
• Misusing brackets: writing NH₄₂C₂O₄ (not acceptable; ammonium must be bracketed as a unit when multiplied).
• Wrong ion ratio: options like NH₄(C₂O₄)₂ would give +1 and −4 overall (not neutral).

Examiner insight: in 1-mark multiple choice items, the distractors are usually built from a single common slip (wrong charge, wrong bracket use, or wrong ratio). Train yourself to charge-check every time.

Do the charges cancel?

• (NH₄)₂ contributes total charge: 2 × (+1) = +2
• C₂O₄ contributes total charge: −2
• Total: +2 + (−2) = 0 ✅

Top-response habit

Always confirm neutrality (total = 0). This is what distinguishes consistent full-mark students on formula/ion questions.