H2SO4 molecular weight is 98g/mol.
out of 98g 64g is due to oxygen
one mole of oxygen is 32g
Then 2moles of oxygen in H2SO4.
If you want to know the mass percent of a substance in a compound.
Simple formula:
Mass percent=Mass of the substance×100/Total mass of the compound
Ex:
Mass percent of oxygen in H2SO4=mass of the oxygen ×100/Total mass of the H2SO4
(i.e 16×4×100/98=65.3061%)
Here oxygen mass is multipled by 4 because there are 4 Oxygen atoms in H2SO4.