Oxidation number of sulphur in ${\mathrm{H}}_2{\mathrm{SO}}_5$ 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.

$$\begin{gathered} 2 \times \left(+ 1\right) + \mathrm{x} + 3 \times \left(-2\right) + 2 \times \left(-1\right) \\ \left(\mathrm{for} \mathrm{H}\right) \left(\mathrm{for} \mathrm{S}\right) \left(\mathrm{for} \mathrm{O}\right) \left(\mathrm{for} \mathrm{O} - \mathrm{O}\right) \\ \Rightarrow 2 + \mathrm{x} - 6 -2 = 0 \\ \Rightarrow \mathrm{x} = 6 \end{gathered}$$