Question

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

• A. $Pb^{2+}$
• B. $Hg^{2+}$
• C. $Cu^{2+}$
• D. $Co^{2+}$

Answer 1

Answer: (B)

$Hg^{2+}$

Answer 2

$\, \, \, \, \, \, \, Hg^{2+} + 2I^- \longrightarrow HgI_2 \, (red)$
$\, \, \, \, \, \, \, HgI_2 + 2KI \longrightarrow K_2 [HgI_4]$
$\, \, \, Hg^{2+} + Co (SCN)_2 \longrightarrow \underset{Blue}{ Co [Hg (SCN)_4]}$