A solution contains non-volatile solute of molecular mass M_2 ....

For dilute solution, the van't Hoff equation is: $$\begin{aligned} \pi & =\frac{n}{\mathrm{~V}} \mathrm{RT} \\ \pi \mathrm{V} & =\frac{m_2}{\mathrm{M}} \mathrm{RT} \\ \Rightarrow \quad \mathrm{M} & =\frac{m_2 \mathrm{RT}}{\pi \mathrm{V}} \end{aligned}$$ Van't Hoff explained that when solutes are dissolved in a solvent they dissociate into ions. Since colligative properties depend only on the number of solute particles, the dissociation of solute molecules into ions results in an increase in the number of particles and hence affects the colligative properties.