A Levels Chemistry (9701)•9701/11/M/J/21
Explanation
Citric acid's acidic groups react with sodium to produce H₂ gas
Steps:
- Identify citric acid formula: C₆H₈O₇, with three -COOH and one tertiary -OH groups.
- Recall reactions: Each -COOH reacts as 2RCOOH + 2Na → 2RCOONa + H₂ (0.5 mol H₂ per -COOH).
- Alcohols react similarly: 2ROH + 2Na → 2RONa + H₂ (0.5 mol H₂ per -OH).
- Total for 1 mol citric acid: 3 × 0.5 + 1 × 0.5 = 2 mol H₂.
Why C is correct:
- Citric acid has four acidic hydrogens (three from -COOH, one from -OH), displacing 2 mol H₂ per mole via the general metal-acid reaction producing H₂ gas.
Why the others are wrong:
- A: Ignores the hydroxyl group, counting only half from one -COOH.
- B: Counts only the three -COOH groups, omitting the -OH.
- D: Overcounts by assuming all eight hydrogens react, but only acidic ones do.
Final answer: C
Topic: Carboxylic acids and derivatives
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