Equilibrium constant expression Steps:

• K_c = [SO₃]² / ([SO₂]² [O₂]) = 280 dm³ mol⁻¹ for the reaction 2SO₂ + O₂ ⇌ 2SO₃.
• Let [SO₃] = x mol dm⁻³ at equilibrium.
• Calculate denominator: [SO₂]² [O₂] = (0.200)² × 0.100 = 0.0400 × 0.100 = 0.00400 mol³ dm⁻⁹.
• x² = 280 × 0.00400 = 1.12 mol² dm⁻⁶, so x = √1.12 ≈ 1.06 mol dm⁻³, which rounds to 1.05 mol dm⁻³.

Why A is correct:

• 1.05 mol dm⁻³ is the value obtained by solving the K_c expression with given equilibrium concentrations, per the definition of K_c.

Why the others are wrong:

• B: 1.20 mol dm⁻³ overestimates [SO₃], as it implies K_c ≈ 720, larger than 280.
• C: 2.36 mol dm⁻³ overestimates greatly, implying K_c ≈ 2780 if using [SO₃] / [SO₂] [O₂] without squares.
• D: 5.60 mol dm⁻³ overestimates extremely, implying K_c = [SO₃] / ([SO₂] [O₂]) = 280, ignoring coefficients.

Final answer: A