A paper moistened with acidified potassium dichromate turns green when exposed to SO2 evolved from sodium sulphite and dilute H2SO4. The balanced change is Cr2O7^2- + 3SO2 + 2H+ -> 2Cr^3+ + 3SO4^2- + H2O. The number of electrons gained per dichromate ion is

JEE Main levelSingle correct optionDifficulty 2 of 3qa170

A paper moistened with acidified potassium dichromate turns green when exposed to $SO X_{2}$ evolved from sodium sulphite and dilute $H X_{2} SO X_{4}$ . The balanced change is $Cr X_{2} O X_{7} X^{2 -} + 3 SO X_{2} + 2 H X^{+} 2 Cr X^{3 +} + 3 SO X_{4} X^{2 -} + H X_{2} O$ . The number of electrons gained per dichromate ion is

Each Cr changes from

+ 6

Cr X_{2} O X_{7} X^{2 -}

+ 3

Cr X^{3 +}

, a gain of 3 electrons per Cr. The ion has two Cr atoms, so

2 \times 3 = 6

electrons. This matches 3

SO X_{2}

losing 2 electrons each (

S

+ 4

+ 6