Oleum is considered as a solution of SO3 in H2SO4. Which is obtained by passing SO3 in solution of H2SO4.When 100 g sample of oleum is diluted with desired weight of H2O then the total mass of H2SO4.obtained after dilution is known as labelling of oleum. For example , a oleum is diluted by 9 g of H2O which combined with all the free SO3 to form H2SO4 as SO3+H2O→H2SO4.