Oxidation number of sulphur in H2SO5 is

+2+x-10=0 ; x=+8, it is not possible to have O.State of sulphur to be greater than +6.Thus the molecule has a peroxy bond. In peroxo bond, O.S of oxygen is -1.Thus two oxygens have -1 state and three oxygens have -2 state.2 × + 1 + x + 3 × -2 + 2 × -1 for H      for S     for O      for O - O ⇒  2 + x - 6 -2 = 0 ⇒ x = 6