Consider separate solution of $0.500{\mathrm{MC}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{aq}\right) 0.100\mathrm{M}{\mathrm{Mg}}_3{\left({\mathrm{PO}}_4\right)}_2\left(aq\right),0.250\mathrm{MKBr}\left(aq\right)$ and $0.125M {\mathrm{Na}}_3{\mathrm{PO}}_4\left(\mathrm{aq}\right)$ at $25°C$.

Which statement is true about these solution, assuming all salts to be strong electrolytes

$\begin{array}{l}0.5{\mathrm{MC}}_2{\mathrm{H}}_5\mathrm{OH}\left(\mathrm{aq}\right)\hspace{0.25em} \hspace{0.25em}\hspace{0.25em}\hspace{0.25em}0.1{\mathrm{MMg}}_3{\left({\mathrm{PO}}_4\right)}_2\left(\mathrm{aq}\right)\\ i=1\hspace{0.25em} i=5\end{array}$

effective molarity $=0.5$  effective molarity $=0.5m$

$\begin{array}{l}0.25\mathrm{MKBr}\left(\mathrm{aq}\right)\hspace{0.25em} \hspace{0.25em} 0.125{\mathrm{MNa}}_3{\mathrm{PO}}_4\left(aq\right)\\ i=2\hspace{0.25em} \hspace{0.25em} \hspace{0.25em}\hspace{0.25em}i=4\end{array}$

effective molarity $=0.5M$  effective molarity $=0.5M$

Hence all colligative properties are same.

Note: This equation is solved by assuming that the examinar has taken $M{g}_3{\left(P{O}_4\right)}_2$ to be completely soluble.

 However the fact is that it is insoluble (sparingly soluble).