Question

$HgC{l}_2$ and $I_2$ both when dissolved in water containing $I^{-}$ ions the pair of species formed is :

• A. $Hg{I}_2,I^{-}$
• B. $Hg{I}_4^{2-},I_3^{-}$
• C. $H{g}_2{I}_2,I^{-}$
• D. $Hg{I}_2,I_3^{-}$

Answer 1

Answer: (B)

In a solution containing $HgC{l}_2,I_2$ and $I^{-}$, both $HgC{l}_2$ and $I_2$ compete for $I^{-}$.

Since formation constant of ${\left[Hg{l}_4\right]}^{2-}$ is $1.9\times 10^{30}$ which is very large as compared with $I_3^{-}\left(K_f=700\right)$

$\therefore I^{-}$ will preferentially combine with $HgC{l}_2$.

$HgC{l}_2+2I^{-}\to \underset{\text{Red ppt}}{Hg{l}_2}\downarrow +2C{l}^{-}$

$Hg{l}_2+2I^{-}\to \underset{\text{soluble}}{{\left[Hg{l}_4\right]}^{2-}}$