Grey-green $Cr (OH) X_{3}$ is warmed with $NaOH$ and $H X_{2} O X_{2}$ , the yellow solution is acidified with acetic acid and lead acetate is added. The change in the oxidation number of chromium in the first step and the final precipitate are

Answer: A

2 Cr (OH) X_{3} + 3 H X_{2} O X_{2} + 4 OH X^{-} 2 CrO X_{4} X^{2 -} + 8 H X_{2} O

raises Cr from

+ 3

+ 6

. In weakly acidic acetic acid the chromate remains

CrO X_{4} X^{2 -}

and gives yellow

PbCrO X_{4}

with

Pb X^{2 +}