JEE Advanced · 2021 · 03 Oct 2021 · Paper 2 · Practical and Analytical Chemistry

The reaction of

K_{3} [Fe (CN)_{6}]

with freshly prepared

FeSO_{4}

solution produces a dark blue precipitate called Turnbull's blue. Reaction of

K_{4} [Fe (CN)_{6}]

with the

FeSO_{4}

solution in complete absence of air produces a white precipitate

X

, which turns blue in air. Mixing the

FeSO_{4}

solution with

NaNO_{3}

, followed by a slow addition of concentrated

H_{2} SO_{4}

through the side of the test tube produces a brown ring.

Precipitate X is

Answer: (C)

Solution

When

K_{4} [Fe (CN)_{6}]

(potassium ferrocyanide) reacts with

FeSO_{4}

in the complete absence of air, Fe²⁺ ions combine with the ferrocyanide complex to form a white precipitate of

K_{2} Fe [Fe (CN)_{6}]

. This white precipitate turns blue upon exposure to air due to oxidation of Fe²⁺ to Fe³⁺, forming Prussian blue. The formula

K_{2} Fe [Fe (CN)_{6}]

represents the white precipitate X.

JEE Advanced · 2021 · 03 Oct 2021 · Paper 2 · Single correct option