$\mathrm{In} \mathrm{acidic} \mathrm{medium}, {\mathrm{H}}_2{\mathrm{O}}_2 \mathrm{changes} {\mathrm{Cr}}_2{\mathrm{O}}_7^{2-} \mathrm{to} {\mathrm{CrO}}_5 \mathrm{which} \mathrm{has}$$\mathrm{two} (-\mathrm{O}-\mathrm{O}-) \mathrm{bonds}.\mathrm{Oxidation} \mathrm{state} \mathrm{of} \mathrm{Cr} \mathrm{in} {\mathrm{CrO}}_5 \mathrm{is}$

${\mathrm{CrO}}_5$has butterfly structure having two peroxo bonds

Peroxo oxygen has -1 oxidation state.

Let oxidation state of Cr be “X”

${\mathrm{CrO}}_5:\mathrm{x}+4(-1)+1(-2)=0\Rightarrow \mathrm{x}=+6$