Write balanced chemical equations to show how `SO_3` is converted to Sulphuric acid in the contact process.

Explanation

Conversion of sulphur trioxide into sulphuric acid :
`underset(underset("trioxide")("Sulphur"))(SO_3) + underset("(conc.)")(H_2SO_4) rarr underset("Oleum")(H_2S_2O_7)`
Dilution of oleum:
`underset("Oleum")(H_2S_2O_7) +H_2O rarr underset("Sulphuric Acid")(2H_2SO_4)`