Question

When mercuric iodide is added to the aqueous solution of potassium iodide the

• A. Freezing point is raised
• B. Freezing point is lowered
• C. Freezing point does not change
• D. Boiling point does not change

Answer 1

Answer: (A)

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
$Hg{I}_2$ although insoluble in water but shows complex formation with KI as
$\text{2KI}\text{+}{\text{HgI}}_{\text{2}}\to {\text{K}}_{\text{2}}\left[{\text{HgI}}_{\text{4}}\right]$
$\text{2KI}\to {\left(\text{2K}\right)}^{\text{+}}{\left(\text{+2I}\right)}^{-}\text{(4}\text{ions)}$
${\left(\text{K}\right)}_{\text{2}}\left[{\left(\text{HgI}\right)}_{\text{4}}\right]\to {\left(\text{2K}\right)}^{\text{+}}{\left(\left[{\left(\text{HgI}\right)}_{\text{4}}\right]\right)}^{-\text{2}}\text{(3}\text{ions)}$
Here number of species decreases due to the reaction. Hence, ${\text{ΔT}}_{\text{f}}\downarrow$ and ${\text{T}}_{\text{f}}\uparrow$