Qualitative Analysis - Result Question 7
####7. A solution of a metal ion when treated with KI gives a red precipitate which dissolves in excess KI to give a colourless solution. Moreover, the solution of metal ion on treatment with a solution of cobalt (II) thiocyanate gives rise to a deep blue crystalline precipitate. The metal ion is
$(2007,3 M)$
(a) $Pb^{2+}$
(b) $Hg^{2+}$
(c) $Cu^{2+}$
(d) $Co^{2+}$
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Answer:
Correct Answer: 7. (b)
Solution:
- $Hg^{2+}+2 I^{-} \longrightarrow HgI _2$ (red)
$$ \begin{aligned} & HgI _2+2 KI \longrightarrow \underset{\text { Soluble }}{K _2\left[HgI _4\right]} \\ & Hg^{2+}+Co(SCN) _2 \longrightarrow \underset{\text { Blue }}{Co\left[Hg(SCN) _4\right]} \end{aligned} $$
