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JEE Main levelSingle correct optionDifficulty 2 of 3qc175

A solution of a salt gives a chocolate-brown precipitate with potassium hexacyanoferrate(II). The precipitate and the oxidation state of iron in it are

A $Cu X_{2} [Fe (CN) X_{6}]$ , Fe in +3
B $Cu X_{2} [Fe (CN) X_{6}]$ , Fe in +2
C $Fe X_{4} [Fe (CN) X_{6}] X_{3}$ , Fe in +2 and +3
D $Zn X_{2} [Fe (CN) X_{6}]$ , Fe in +2

Answer: B

Chocolate brown

Cu X_{2} [Fe (CN) X_{6}]

is the test for

Cu X^{2 +}

. Charge balance: two

Cu X^{2 +}

(+4) neutralise

[Fe (CN) X_{6}] X^{4 -}

, so Fe is +2. Zinc gives a bluish-white, not brown, precipitate.

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