Percentage Labeling of Oleum
Oleum contains H2SO4 and SO3 only. When oleum is diluted (by adding water), SO3 reacts with H2O to form H2SO4, thus increasing the mass of the solution.
The total mass of H2SO4 obtained by diluting 100 g of oleum sample with the required amount of water is equal to the percentage labeling of oleum. Therefore,
Percentage labeling of oleum
= Total mass of H2SO4 present in oleum after dilution
= Mass of H2SO4 initially present + Mass of H2SO4 produced on dilution