A solution of a metal ion when treated with KI gives a red...

A solution of a metal ion when treated with

\mathrm{KI}

gives a red precipitate which dissolved 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 precipitqte. The metal ion is

Options

A $\mathrm{Pb}^{2+}$
B $\mathrm{Hg}^{2+}$
C $\mathrm{Cu}^{2+}$
D $\mathrm{Co}^2$

Correct answer

B. $\mathrm{Hg}^{2+}$

Step-by-step solution

\begin{aligned} & \mathrm{Hg}^{2+}+2 \mathrm{I}^{-} \longrightarrow \mathrm{HgI}_2, \mathrm{HgI}_2+\mathrm{H}^{-} \\ & \text{(Excess) }\left.\longrightarrow \mathrm{HgI}_4\right]^{2-} \\ & \text{ Soluble }^{2-} \\ & \mathrm{Hg}^{2+}+\mathrm{Co}(\mathrm{SCN})_2 \longrightarrow \mathrm{Hg}(\mathrm{SCN})_2 \end{aligned} Blue crystalline ppt.

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