i =

$$\frac{\text{colligative property of electrolyte solution}}{\text{colligative property of nonelectrolyte solution of the same concentration}}$$

$$= \frac{\Delta T_f}{\Delta T_{f_0}} = \frac{\Delta T_b}{\Delta T_{b_0}} = \frac{\Delta P}{\Delta P_0} = \frac{\pi}{\pi_0}$$