A Levels Chemistry (9701)•9701/12/M/J/21
Explanation
Equilibrium constant from ICE table Steps:
- Set up ICE table: Initial H₂ = 3.6 mol, I₂ = 2.0 mol, HI = 0 mol.
- Moles of H₂ reacted = 3.6 - 3.20 = 0.40 mol (let x = 0.40).
- Equilibrium moles: H₂ = 3.20 mol, I₂ = 2.0 - 0.40 = 1.60 mol, HI = 2 × 0.40 = 0.80 mol.
- K_c = (n_{HI})^2 / (n_{H_2} n_{I_2}) = (0.80)^2 / (3.20 × 1.60) = 0.64 / 5.12 = 0.125 (moles proportional to concentrations since V cancels).
Why B is correct:
- Matches K_c = 0.125 from stoichiometry and equilibrium mole ratios for Δn = 0 reaction.
Why the others are wrong:
- A: Underestimates HI production, ignoring full 2:1 stoichiometry.
- C: Overestimates by misapplying x to only one product.
- D: Assumes near-complete reaction, inverting the small K_c value.
Final answer: B
Topic: Equilibria
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