During the qualitative analysis of $\mathrm{SO}_{3}^{2-}$ using dilute $\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{SO}_{2}$ gas is evolved which turns $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ solution (acidified with dilute $\mathrm{H}_{2} \mathrm{SO}_{4}$ ):

JEE Main · 2023 · 30 Jan 2023 · Shift 1 · Practical and Analytical Chemistry

During the qualitative analysis of

SO_{3}^{2 -}

using dilute

H_{2} SO_{4}, SO_{2}

gas is evolved which turns

K_{2} Cr_{2} O_{7}

solution (acidified with dilute

H_{2} SO_{4}

(A)Black
(B)Red
(C)Green✓ Correct
(D)Blue

Answer: (C)

Solution

Sulphite ion treated with dilute sulphuric acid liberates sulphur dioxide, a good reducing agent. Passed into the orange acidified dichromate solution, it reduces chromium from the

+ 6

state in

C r_{2} O_{7}^{2 -}

to the

+ 3

state as

C r^{3 +}

, while itself being oxidised to sulphate. The chromium(III) ion gives a green colour, so the acidified dichromate solution turns green.

JEE Main · 2023 · 30 Jan 2023 · Shift 1 · Single correct option

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