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NEET levelSingle correct optionDifficulty 1 of 3qa148

The brown ring formed in the test for nitrate is due to the complex ion:

A $[Fe (H X_{2} O) X_{5} NO] X^{2 +}$
B $[Fe (CN) X_{6}] X^{4 -}$
C $[Fe (CN) X_{5} NO] X^{2 -}$
D $[Fe (H X_{2} O) X_{6}] X^{3 +}$

Answer: A

Nitrate is reduced to NO by

Fe X^{2 +}

in conc.

H X_{2} SO X_{4}

, and NO displaces one water:

[Fe (H X_{2} O) X_{6}] X^{2 +} + NO [Fe (H X_{2} O) X_{5} NO] X^{2 +} + H X_{2} O

, which forms the brown ring.

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