$\underset{ (\mathrm{solid}) }{{\mathrm{HgI}}_2}+\underset{ (\mathrm{aq}) }{2\mathrm{KI}}\to {\mathrm{K}}_2\left[{\mathrm{Hgl}}_4\right](100\% \mathrm{yield})$

If exactly sufficient HgI₂ solid is dissolved in an aqueous potassium iodide solution, which of the following may be observed? (all substances are completely ionised)

${\mathrm{HgI}}_{2\left(\mathrm{s}\right)}+2{\mathrm{KI}}_{\left(\mathrm{aq}\right)}\to {\mathrm{K}}_2\left({\mathrm{HgI}}_4\right)$
1 mole   2 moles     1 mole
(4 mole particle) (3 mole particle)
4 mole particle changes to 3 moles particles
&   ↓ by 1mole particle so 25% of molarity ↓ '${\mathrm{\Delta T}}_{\mathrm{f}}$' of KI solution decreases by 25%