This is not something to guess, you can calculate from the reaction equation.
Assume you have initially - say - 1 mole of SO2
and 5 moles of O2
. There is an excess of oxygen. How many moles of both gases together?
Now reaction proceeds to the end. 1 mole of SO2
reacted, there is none left. How many moles of SO3
were produced? How many moles of O2
left? How many moles of both gases together?
What is the difference?
Does this difference depend on the number of moles of SO2
that reacted? If you are not sure - do identical calculations for an initial mixture of 0.5 moles SO2
and 5 moles of oxygen. For 2 moles SO2
in 5 moles of oxygen. Actually any number of moles of SO2
in excess number of moles of oxygen. Do you see a pattern here?
If so, knowing how many moles of the mixture were present the reaction and after the reaction, can you calculate how many moles of SO2
were present initially in the mixture?