umm.....

118 gms is actually the total weight of pure h2so4

that will be present when we add water to 100 gm of oleum...rit.....so that it gets converted to h2so4

so h20 +so3  ----   h2so4

that means 1 mole h2o i.e 18 gmcombines with 1 mole so3 i.e 80gm...clear..

so,    ( 18 gm h20  ) .   (  80gm so3/18gm h20)

= 80

so 100 gm oleum contains 80 gm so3...

or 80%

hope that does it

cheers