Question

$A$ is a colorless substance. Aq. solution of $A$ gives reddish-orange ppt. with $KI$; ppt. dissolves in excess of $KI$ forming a colorless solution. If $N{H}_{4}Cl$ and $NaOH$ solution is added to this colorless solution, reddish brown ppt. is formed. Substance $A$ is:

• A. Epsom salt
• B. Mohr's salt
• C. Calomel
• D. Corrosive sublimate

Answer 1

The correct option is D Corrosive sublimate

A is Mercury(II) chloride or mercuric chloride (archaically, corrosive sublimate) is the chemical compound of mercury and chlorine with the formula $HgC{l}_2$. This white crystalline solid is a laboratory reagent and a molecular compound.

$HgC{l}_2\left(aq\right)+2KI\to H{g}_2{I}_2$ (reddish - orange ppt) $+2KCl$

$H{g}_2{I}_2+$ excess $KI\to K_{2}Hg{I}_4$ ( colorless solution)

Now, on reaction with $N{H}_{4}Cl$ and $NaOH$ solution, this colorless solution, changes to brown ppt.

$N{H}_4^{+}+2[Hg{I}_4{]}^{2-}+4O{H}^{-}\to HgO\cdot Hg(N{H}_2)I\downarrow +7I^{-}+3H_{2}O$