A green solution of Cr^3+ is made strongly alkaline and warmed with H2O2 until the colour turns yellow. How many moles of H2O2 are consumed per mole of Cr^3+?

JEE Main levelSingle correct optionDifficulty 2 of 3qc138

A green solution of $Cr X^{3 +}$ is made strongly alkaline and warmed with $H X_{2} O X_{2}$ until the colour turns yellow. How many moles of $H X_{2} O X_{2}$ are consumed per mole of $Cr X^{3 +}$ ?

Cr X^{3 +}

is oxidised to

CrO X_{4} X^{2 -}

(

+ 3

+ 6

, 3 electrons). Each

H X_{2} O X_{2}

accepts 2 electrons, so the balanced equation is

2 Cr X^{3 +} + 3 H X_{2} O X_{2} + 10 OH X^{-} 2 CrO X_{4} X^{2 -} + 8 H X_{2} O

. This is 3 mol

H X_{2} O X_{2}

per 2 mol Cr, i.e. 1.5 mol per mole of

Cr X^{3 +}