Question

An aqueous blue coloured solution of a transition metal sulphate reacts with $H_{2}S$ to give a black precipitate. The black precipitate dissolves in $50\%$ nitric acid forming a blue coloured solution. The blue solution on treatment with $KI$ in weekly acidic medium turns yellow brown and produce a white precipitate. The cation of blue coloured solution is :

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

Answer 1

Answer: (B)

$C{u}^{2+}$

$CuS{O}_4$ is a blue color solution and when it reacts with $H_{2}S$, a black precipitate of $CuS$ is formed.
$CuS{O}_4+H_{2}S\to CuS\left(blackppt\right)+H_{2}S{O}_4$
When Copper Suphide is dissolved in nitric acid, a blue colored solution of copper nitrate is formed.
$3CuS+8HN{O}_3\to Cu\left(N{O}_3{)}_2\right(bluesolution)+2NO+3S+4H_{2}O$
This blue colored copper nitrate solution when treated with $KI$ in weak acidic medium, a white precipitate of cupric iodine is formed and color of the solution becomes yellow brown.
$2C{u}^{2+}+4I^{-}\to C{u}_2{I}_2\downarrow \left(whiteppt\right)+I_2\left(yellowbrownsolution\right)$