Acidified chromate solution is shaken with H2O2 and ether, giving a blue colour in the ether layer, due to CrO5 (one oxo, two peroxo groups). The oxidation state of chromium and the nature of the change are

+6; no change in oxidation state of Cr (peroxide complex formation)

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc093

Acidified chromate solution is shaken with $H X_{2} O X_{2}$ and ether, giving a blue colour in the ether layer, due to $CrO X_{5}$ (one oxo, two peroxo groups). The oxidation state of chromium and the nature of the change are

Taking O

= - 2

for the oxo oxygen and

- 1

for each of the four peroxo oxygens,

x - 2 - 4 = 0

gives

x = + 6

. Chromium(VI) stays

+ 6

; the blue colour is a ligand-to-metal charge transfer in the peroxo complex, which is unstable and decomposes.