Excess and limiting reactant problem

Im having a dispute with friends about this question

4HCl(g) + O2(g) --> 2H2O(l) + 2Cl2 (g)
All gases are measured at constant temperature and pressure.
If 40.0 mL of HCl reacts with 30.0 mL of O2 which gas is in excess and by how much? What is the volume of the final gaseous mixture?

O2 is in excess by 20.0 mL
Final volume of gases (20.0 mL O2 + 20.0 mL Cl2) = 40.0 mL