how many oxygen molecules are in 9.1 liters of oxygen gas?
Technically any answer will do since you didn't specify T and P.
There are 6.02E23 molecules in a mole of anything and 1 mol O2 occupies 22.4L at STP
So mols O2 = 9.1L/22.4L = ?
Then ? x 6.02E23 = # molecules.