How many liters of chlorine gas can be produced when 0.98 l of hcl react with excess o2 at stp? 4hcl(g) + o2(g) -> 2cl2(g) + 2h2o(g)?

The balanced equation for the reaction is as follows;
4HCl + O₂ ---> 2Cl₂ + 2H₂O
stoichiometry of HCl to Cl₂ is 4:2
Avagadros law states that volume is proportional to number of moles of gas when pressure and temperature are constant 
therefore volume ratio is equal to the molar ratio
therefore molar ratio of HCl to Cl₂ is 4:2
volume of HCl reacted - 0.98 L
therefore volume of Cl₂ formed - 0.98 L / 2 = 0.49 L

The mass has escaped into a gas which you can not see so the law stays true.