Example Question - 3

Question: 0.400 mol of SO₂ and 0.200 mol of O₂ were placed in a 2.00 dm³ vessel. At equilibrium, 0.300 mol of SO₃ had formed. Calculate K_c and give its units.
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)Step 1: equilibrium moles. Forming 0.300 mol of SO₃ uses 0.300 mol of SO₂ and 0.150 mol of O₂. SO₂ left = 0.100 mol and O₂ left = 0.050 mol.Step 2: equilibrium concentrations. Divide each by 2.00 dm³: [SO₂] = 0.0500, [O₂] = 0.0250 and [SO₃] = 0.150 mol dm^-3. The volume does not cancel here because there are more concentration terms on the bottom.Step 3: K_c = [SO₃]² / [SO₂]²[O₂] = 0.150² / (0.0500² × 0.0250) = 360Step 4: units. (mol dm^-3)² / (mol dm^-3)³ = (mol dm^-3)^-1, so K_c = 360 mol^-1 dm³.