For the equilibrium H2(g) + I2(g) <=> 2HI(g), the equilibrium concentrations are [H2] = 0.20 mol/dm3, [I2] = 0.20 mol/dm3 and [HI] = 1.6 mol/dm3. Calculate Kc.
[3 marks]
Total for this question: 3
All calculations
Equilibrium constant Kc
Physical chemistry
AQA 7405
ORIGINAL
Kc = [products] / [reactants], each concentration raised to the power of its balancing number (at equilibrium)
Products over reactants, each raised to its balancing number — and the units come from the powers.
Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Where the marks get lost
- Putting reactants over products — it is always products on top.
- Forgetting to raise each concentration to the power of its balancing number.
- Quoting units without working them out — here they cancel, so Kc has none.
Exam tip: Write the Kc expression straight from the balanced equation first, then substitute. Work the units from the powers: (mol/dm3) to the (total product powers minus total reactant powers).
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