Question

Why so3 is dissolved in concentrated h2so4 and not in water in the contact process?

Answer 1

SO3 dissolution in water is extremely exothermic... when dissolved in water sulphur trioxide forms a mist of sulphuric acid which can be difficult to store and handle..
bcoz of this in order to form sulphuric acid an intermediate called oleum must be formed where SO3 is dissolved in H2SO4 giving H2S2O7.