2 SO2 (g) + O2 (g) → 2 SO3­↑

Question

Assuming STP, how many grams of oxygen are needed to produce 19.8 liters of SO3?

Answer 1

When gases are involved you may use a shortcut; i.e., use volumes as if volume = mols.
19.8 L SO3 x (1 mol O2/2 mols SO3) = ? L O2.
Then grams = mols x molar mass.

Answer 2

02+2SO2=2SO3
volume(O2)=19.8L*(1LO2/2LSO3)
=9.9L of O2