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Q.
When mercuric iodide is added to the aqueous solution of potassium iodide the
NTA AbhyasNTA Abhyas 2022
Solution:
When mercuric iodide is added to the aqueous solution of potassium iodide the freezing point is raised as $\Delta T_{f}$ decreases due to complex formation number of species decrease and molecular weight increases.
$\text{ΔT}_{\text{f}} \propto \frac{1}{\text{Molecular} \, \text{weight}} \propto $ number of species
$HgI_{2}$ although insoluble in water but shows complex formation with KI as
$\text{2KI} \, \text{+} \, \text{HgI}_{\text{2}} \rightarrow \text{K}_{\text{2}} \left[\text{HgI}_{\text{4}}\right]$
$\text{2KI} \rightarrow \left(\text{2K}\right)^{\text{+}} \left(\text{+2I}\right)^{-} \, \text{(4} \, \text{ions)}$
$\left(\text{K}\right)_{\text{2}} \left[\left(\text{HgI}\right)_{\text{4}}\right] \rightarrow \left(\text{2K}\right)^{\text{+}} \left(\left[\left(\text{HgI}\right)_{\text{4}}\right]\right)^{- \text{2}} \, \text{(3} \, \text{ions)}$
Here number of species decreases due to the reaction. Hence, $\text{ΔT}_{\text{f}} \downarrow$ and $\text{T}_{\text{f}} \uparrow$