Gas volumes follow reaction stoichiometry

Steps:

• Balanced equation: 2SO₂ + O₂ → 2SO₃ shows 2 volumes SO₂ react with 1 volume O₂ to produce 2 volumes SO₃.
• 3 dm³ SO₂ requires 1.5 dm³ O₂ (3/2 × 1), but 2 dm³ O₂ is available, so SO₂ is limiting.
• 3 dm³ SO₂ produces 3 dm³ SO₃ (same volume as SO₂, per 2:2 ratio).
• O₂ excess (0.5 dm³ unused) does not affect SO₃ volume.

Why B is correct:

• Avogadro's law states equal volumes of gases at same T/P contain equal moles, so product volume matches stoichiometric ratio to limiting reactant.

Why the others are wrong:

• A: Ignores 2:2 ratio, underestimates SO₃ from SO₂.
• C: Assumes full reaction of both, adds volumes incorrectly.
• D: Overestimates by summing all reactant volumes.

Final answer: B