Question

A solution of a metal ion, when treated with KI, gives a red precipitate which dissolves in an excess of KI to give a colourless solution. Moreover, the solution of the metal ion on treatment with a solution of cobalt (II) thiocyanate gives rise to a deep blue crystalline precipitate. The metal ions is:

• A. $P{b}^{2+}$
• B. $H{g}^{2+}$
• C. $C{u}^{+}$
• D. $C{u}^{2+}$

Answer 1

The correct option is B $H{g}^{2+}$

$H{g}^{2+}$ ions react with KI to give a red precipitate of $Hg{I}_2$ which dissolves in an excess of the reagent.

$H{g}^{2+}+2I^{-}\to Hg{I}_2\downarrow \stackrel{2I^{-}}{\to }[Hg{I}_4{]}^{2-}$

(red) (colourless)

$H{g}^{2+}$ ions react with cobalt (II) thiocyanate to give a blue precipitate of $Hg\left[Co\right(SCN{)}_4]$, also formulated as $Co\left[Hg\right(SCN{)}_4]$.

$H{g}^{2+}+C{o}^{2+}+4SC{N}^{-}\to Hg\left[Co\right(SCN{)}_4]\downarrow$

(blue)