Dear student,

98% by weight H2SO4 means 98gm of H2SO4 is present in 100 gm of solution.

The solution consists of solvent and solute. Here, since 98gm is of H2SO4 then 2gm is water ( if nothing is given then assume by default water as one of the solvent or solute as per the requirement ).

So, 98gm H2SO4 is = 1mol.

And 2 gm water is = 2/18 = 0.11mol

So, mole fraction of H2SO4 is = 1/(1+0.11) = 0.9009