Colligative properties depend on the number of dissolved particles, and that electrolytes must be treated slightly differently than nonelectolytes when determining colligative properties.
For example, consider a 0.10 m sucrose solution. The freezing point depression of this is 0.186 degrees Celsius, whereas a 0.10 m sodium chloride solution is nearly twice of that. This is because 1 mol of sodium cholride dissosicates to form nearly 2 mol of ions in solution.
This is where van't Hoff factor comes into play.
The van't Hoff factor (i) is the ratio of moles of particles in solution to moles of formula units dissolved,
i = moles of particles in solution / moles of formula units dissolved