Chemistry: How does the volume change if you mix 3L of SO2 and 2L of O2 and they produce SO3?

Qualitative answer :

The equation for the reaction (assuming the reaction completes):
2SO₂(g) + O₂(g) → 2SO₃(g)

In the equation, there are 3 moles of gas molecules on the left and 2 moles of the right. The number of moles of gas molecules is reduced, and this leads to the DECREASE IN VOLUME under constant temperature and pressure.


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Quantitative answer :

The equation for the reaction (assuming the reaction completes):
2SO₂(g) + O₂(g) → 2SO₃(g)
Volume ratio (under constant temperature and pressure) SO₂ : O₂ : SO₃ = 2 : 1 : 2

If 3 L of SO₂ completely reacts, O₂ needed = (3 L) × (1/2) = 1.5 L < 2 L
O₂ is in excess.
Hence, SO₂ is in the limiting reactant/reagent.

Volume of SO₂ reacted = 3 L
Volume of O₂ reacted = (3 L) × (1/2) = 1.5 L
Volume of SO₃ formed = (3 L) × (2/2) = 3 L

Initial volume = (3 L) + (2 L) + (0 L) = 5 L
Final volume = [(3 - 3) L] + [(2 - 1.5) L] + (3 L) = 3.5 L

The volume of change from 5 L to 3.5 L.
OR: The volume is reduced by 1.5 L