Acidified potassium dichromate solution (orange) turns green when SO2 is passed through it: Cr2O7^2- + 3SO2 + 2H+ -> 2Cr^3+ + 3SO4^2- + H2O. How many moles of SO2 are oxidised by one mole of dichromate?

JEE Main levelSingle correct optionDifficulty 2 of 3qa023

Acidified potassium dichromate solution (orange) turns green when $SO X_{2}$ is passed through it: $Cr X_{2} O X_{7} X^{2 -} + 3 SO X_{2} + 2 H X^{+} 2 Cr X^{3 +} + 3 SO X_{4} X^{2 -} + H X_{2} O$ . How many moles of $SO X_{2}$ are oxidised by one mole of dichromate?

Each Cr goes from +6 to +3, so the dichromate takes up 6 electrons. Each

SO X_{2}

(

S

: +4 to +6) gives 2 electrons, so 6/2 = 3 mol of

SO X_{2}

are oxidised. The balanced equation confirms this ratio.