Question

Addition of $SnC{l}_2$ to $HgC{l}_2$ gives precipitate

• A. white turning to red
• B. white turning to grey
• C. black turning to white
• D. None of the above

Answer 1

Answer: (B)

Stannous chloride $\left(SnC{l}_2\right)$ is a good reducing agent. It reduces mercuric chloride $\left(HgC{l}_2\right)$ to white ppt. of mercurous chloride $\left(H{g}_{2}C{l}_2\right)$ and then reduces $H{g}_{2}C{l}_2$ into grey ppt. of mercury. The following reaction takes place in two steps.
$SnC{l}_2+HgC{l}_2⟶SnC{l}_4+\underset{\text{ white ppt. }}{H{g}_{2}C{l}_2}\downarrow$
$SnC{l}_2+H{g}_{2}C{l}_2⟶SnC{l}_4+\underset{\text{ grey ppt. }}{2Hg}\downarrow$
Therefore, addition of $SnC{l}_2$ to $HgC{l}_2$ gives white $ppt$. turning to grey $ppt$.