Question

$HgCl_2$ and $I_2$ both when dissolved in water containing $I^-$ ions the pair of species formed is :

• A. $HgI_2 , I^-$
• B. $HgI_4^{2-} , I^-_3$
• C. $Hg_2I_2 , I^-$
• D. $HgI_2 , I^{-}_3$

Answer 1

Answer: (B)

$HgI_4^{2-} , I^-_3$

Answer 2

In a solution containing $HgCl _{2}, I _{2}$ and $I ^{-}$, both $HgCl _{2}$ and $I _{2}$ compete for $I ^{-}$.

Since formation constant of $\left[ Hgl _{4}\right]^{2-}$ 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 $HgCl _{2}$.

$HgCl _{2}+2 I ^{-} \rightarrow \underset{\text{Red ppt}}{Hgl _{2}} \downarrow+2 Cl ^{-}$
$Hgl _{2}+2 I ^{-} \rightarrow \underset{\text{soluble}}{\left[ Hgl _{4}\right]^{2-}}$