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AS · Practice questions · The equilibrium constant Kp

Share out the pressure.

Six original Cambridge-style questions on Kp: partial pressures, expressions, units, calculations from equilibrium amounts and the conditions that change Kp.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Moles, fractions, pressures, then Kp.

01
Recall
[2 marks]

Define partial pressure and state how it is calculated.

  • The pressure a gas in a mixture would exert if it alone occupied the whole container. ✓
  • Partial pressure = mole fraction × total pressure. ✓
02
Recall
[2 marks]

Write the Kp expression and units (in Pa) for N2(g) + 3H2(g) ⇌ 2NH3(g).

  • Kp = p(NH3)2 ÷ [p(N2) × p(H2)3]. ✓
  • Units: Pa2 ÷ Pa4 = Pa−2. ✓
03
Calculation
[4 marks]

1.00 mol of N2O4 is allowed to reach equilibrium at a total pressure of 100 kPa; 20.0% dissociates into NO2. Calculate Kp.

  • Equilibrium moles: N2O4 0.800, NO2 0.400; total 1.20. ✓
  • Mole fractions: 0.667 and 0.333. ✓
  • Partial pressures: 66.7 kPa and 33.3 kPa. ✓
  • Kp = 33.32 ÷ 66.7 = 16.7 kPa. ✓
04
Calculation
[2 marks]

At equilibrium p(SO2) = 20 kPa, p(O2) = 10 kPa and p(SO3) = 70 kPa. Calculate Kp for 2SO2 + O2 ⇌ 2SO3 and give its units.

  • Kp = 702 ÷ (202 × 10) = 4900 ÷ 4000. ✓
  • Kp = 1.23 kPa−1. ✓
05
Explain
[2 marks]

For N2O4 ⇌ 2NO2, ΔH = +58 kJ mol−1. State and explain the effect of increasing the temperature on Kp.

  • The forward reaction is endothermic, so the position shifts right on heating. ✓
  • p(NO2) increases and p(N2O4) decreases, so Kp increases. ✓
06
Explain
[2 marks]

Explain why increasing the total pressure on the N2O4/NO2 equilibrium changes the composition of the mixture but not Kp.

  • Higher pressure shifts the position to the left, towards fewer gas molecules (1 mol rather than 2). ✓
  • The partial pressures readjust until p(NO2)2 ÷ p(N2O4) equals the same Kp, which depends only on temperature. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.