In an experiment a volume of 300cm3 of oxygen was used but only 10% of oxygen was converted into ozone all volumes were measured at the same temperature and pressure the total volume of gas present in cm3 was:
270cm3 of O2
and unknown volume of Ozone from 30 cm3 of O2.
Consider as Ozone is O3, we must have 1.5x (3/2) the volume of O2 as that's how many molecules there would be of ozone to use up the same amount of individual Oxygen atoms.
Think if we had 10 molecules of O3 we would have 30 oxygen atoms. This could make 15 O2 molecules. This is 1.5x our original amount of atoms.
30/1.5 gives us 20cm3
Add the 270cm3 of Oxygen gives 290cm3 total volume.