Question

How many liters of chlorine gas can be produced when 1.08 L of HCl react with excess O2 at STP?
4HCL(g) + O2(g) à 2Cl2(g) + 2H2O(g)

Answer 1

In STP n = V / 22.41 litres /mol,
But there I no need to calculate moles.

Volumes of gases are in relation of number of moles.
If 4 moles of HCl gives 2 moles of Cl2,
How it is with volumes?

Answer 2

You're converting the HCl to Cl2. the fact that there's excess O2 simply means that you can convert this directly (you'll learn about excess later on).