NEET 2023, Chemistry Q15: Principles Of Qualitative Analysis

NEET2023Chemistry

In Lassaigne's extract of an organic compound, both nitrogen and sulphur are present, which gives blood red colour with Fe $^{3 +}$ due to the formation of:

A
NaSCN
B
$[Fe (CN)_{5} NOS]^{4 -}$
C
$[Fe (SCN)]^{2 +}$
D
$Fe_{4} [Fe (CN)_{6}]_{3} \cdot x H_{2} O$

Solution

When both N and S are present in the organic compound, fusion with sodium gives sodium thiocyanate: $Na + C + N + S \to NaSCN$ .

On treatment with $Fe^{3 +}$ , the thiocyanate ion forms the blood-red complex:

$Fe^{3 +} + SCN^{-} \to [Fe (SCN)]^{2 +}$ (blood red).

No Prussian blue forms because no free cyanide is available.

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NEET 2023, Chemistry Q15: Principles Of Qualitative Analysis

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