The balanced chemical reaction would be :
S + O2 = SO2
2SO2 + O2 = 2SO3
We are given the amount of oxygen to be used in the reaction. This will be the starting value in the calculations. We do as follows:
2.56 L O2 ( 1 mol / 22.4 L) ( 1 mol SO2 / 1 mol O2 ) ( 2 mol SO3 / 2 mol SO2 ) 22.4 L / 1 mol = 2.56 mol SO3