A green solution of a chromium(III) salt is treated with excess NaOH and H2O2 and warmed; the yellow solution is acidified with acetic acid and lead acetate is added. In a separate test, acidified H2O2 with the yellow ion gives a blue peroxo compound CrO5 (one oxo and two peroxo groups). The yellow ion, the yellow precipitate and the oxidation state of Cr in CrO5 are

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc196

A green solution of a chromium(III) salt is treated with excess NaOH and $H X_{2} O X_{2}$ and warmed; the yellow solution is acidified with acetic acid and lead acetate is added. In a separate test, acidified $H X_{2} O X_{2}$ with the yellow ion gives a blue peroxo compound $CrO X_{5}$ (one oxo and two peroxo groups). The yellow ion, the yellow precipitate and the oxidation state of Cr in $CrO X_{5}$ are

Alkaline

H X_{2} O X_{2}

oxidises Cr(III) to yellow chromate;

Pb X^{2 +}

then gives yellow

PbCrO X_{4}

. In

CrO X_{5}

the two peroxo groups are

O X_{2} X^{2 -}

(4 O at

- 1

each) and one oxide:

x + 4 (- 1) + (- 2) = 0

, so

x = + 6

(+10 results from treating all five O as

- 2