Qualitative Analysis - Result Question 37
Passage
When a metal rod $M$ is dipped into an aqueous colourless concentrated solution of compound $N$, the solution turns light blue. Addition of aqueous $NaCl$ to the blue solution gives a white precipitate $O$. Addition of aqueous $\mathrm{NH}_3$ dissolves $O$ and gives an intense blue solution.
(2011)
40. The final solution contains
(a) $\left[Pb\left(NH _3\right) _4\right]^{2+}$ and $\left[CoCl _4\right]^{2-}$
(b) $\left[Al\left(NH _3\right) _4\right]^{3+}$ and $\left[Cu\left(NH _3\right) _4\right]^{2+}$
(c) $\left[Ag\left(NH _3\right) _2\right]^{+}$and $\left[Cu\left(NH _3\right) _4\right]^{2+}$
(d) $\left[Ag\left(NH _3\right) _2\right]^{+}$and $\left[Ni\left(NH _3\right) _6\right]^{2+}$
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Answer:
Correct Answer: 40. (c)
Solution:
Since, in the beginning, concentrated $AgNO _3$ solution was taken, some $AgNO _3$ remain in solution which gives white precipitate of $AgCl$ on addition of $NaCl$. The precipitate finally dissolve in ammonia and $Cu^{2+}$ present in solution also forms complex with ammonia as
$ Cu\left(NO _3\right) _2+4 NH _3(a q) \rightarrow \underset{\text {Blue }} {[Cu\left(NH _3\right) _4]^{2+}}+2 NO _3^{-} $
$ AgCl(s)+2 NH _3(a q) \rightarrow\left[Ag\left(NH _3\right) _2\right]^{+}+Cl^{-}$
The comprehension describing methylene-blue test.
BETA
