In acidic medium, H2O2 changes Cr2O72- to CrO5 which has two (-O-O-) bonds.Oxidation state of Cr in CrO5 is
CrO5has butterfly structure having two peroxo bonds
Peroxo oxygen has -1 oxidation state.
Let oxidation state of Cr be “X”
CrO5:x+4(-1)+1(-2)=0⇒x=+6