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Gas phase limiting reactant problem....

Balance the equation first:
4HCl(g) + O2(g) → 2Cl2(g) + 2H2O(g)
50.0L ...... 10.4L ..... ?L

Thanks to Avogadro and Gay-Lussac (law of combining volumes) we know that the volumes of gases in a reaction at the same temperature and pressure are in the mole ratio given by the balanced chemical equation.

If 10.4L of O2 reacts then 41.6L of HCl are required according to the 4:1 mole ratio of HCl to O2. This indicates that HCl is in excess and O2 is the limiting reactant. Therefore, 20.8L of Cl2 is produced. The math is easy enough to do in your head.

At constant temperature and pressure, volume ratio is equal to mole ratio.

HCl(g) + O₂(g) → Cl₂(g) + H₂O(g)
Volume ratio in reaction HCl : O₂ = 1 : 1

(Volume of HCl) > (Volume of O₂)
HCl is in excess and O₂ completely reacts.

According to the above equation, volume ratio in reaction O₂ : Cl₂ = 1 : 1
Volume of Cl₂ produced = Volume of O₂ reacted = 10.4 L

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