Colligative Properties and Determination of Molar Mass

When a non-volatile solute is dissolved in a volatile solvent, the vapour pressure of the solvent falls (as established in the study of vapour pressure of solid-in-liquid solutions). Several measurable properties of the solution are tied directly to this drop in vapour pressure.

What makes a property "colligative"

The striking feature of these properties is that their magnitude depends only on how many solute particles are dissolved — not on what those particles are. A mole of glucose and a mole of urea, dissolved separately in the same amount of the same solvent, produce the same effect, because each contributes the same number of particles. …