In the manufacture of sulphuric acid(contact process) to make oleum sulphuric acid is used. why is this so? why is sulphuric acid used to make sulphuric acid? what is the gain in doing this? 

Hi,

Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum.

When SO₃ is added to water, rather than simply dissolving, it tends to form a fine mist of sulfuric acid, which is difficult to manage and thus we use concentrated sulfuric acid to absorb the SO₃ gas. However, SO₃ added to concentrated sulfuric acid readily dissolves, forming oleum which can then be diluted with water to produce additional concentrated sulfuric acid.