Exam-Style Questions - 3

Question. Phosphorus(V) chloride decomposes on heating:PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)A sealed 2.0 dm³ container is found to contain 0.40 mol PCl₅, 0.20 mol PCl₃ and 0.20 mol Cl₂ at equilibrium at a temperature T.(a) Write the expression for K_c. (1)
(b) Calculate K_c at temperature T, giving its units. (3)Model answer.(a) K_c = [PCl₃][Cl₂] / [PCl₅](b) First convert each amount to a concentration using c = n ÷ V:[PCl₅] = 0.40 ÷ 2.0 = 0.20 mol dm⁻³
[PCl₃] = 0.20 ÷ 2.0 = 0.10 mol dm⁻³
[Cl₂] = 0.20 ÷ 2.0 = 0.10 mol dm⁻³K_c = (0.10 × 0.10) / 0.20 = 0.050 mol dm⁻³Examiner's comments.• The most common error is putting moles straight into the expression. Here that gives (0.20 × 0.20) / 0.40 = 0.10, which is wrong by a factor of 2. Moles can only be used directly when the units of K_c cancel, so the volume has no effect.
• The values used must be the equilibrium amounts. If you are given starting amounts, you must first work out how much has reacted.
• Give the units: (mol dm⁻³)(mol dm⁻³) / mol dm⁻³ = mol dm⁻³.
• Give the answer to 2 significant figures, to match the data.