Colligative Properties

Lesson

Part 1 — three effects of a non-volatile solute that depend only on how many particles it supplies.

In this lesson you will

Explain why colligative properties depend on the number, not the nature, of solute particles
Calculate molar mass from the relative lowering of vapour pressure
Derive the expressions for elevation of boiling point and depression of freezing point
Calculate molar mass from boiling point elevation and freezing point depression

Concept

What “colligative” means

Colligative means “bound together”: all four effects depend on the number of solute particles per amount of solvent. One mole of glucose and one mole of urea in the same mass of water give the same effects; one mole of NaCl, which gives two moles of ions, gives about twice the effect.

Table comparing the number of particles and relative colligative effect of four solutes
Solute in 1 kg water	Moles of particles	Relative effect
1 mol glucose	1	1
1 mol urea	1	1
1 mol KCl (fully dissociated)	2	about 2
1 mol Na₂SO₄ (fully dissociated)	3	about 3

Diagram

Lowering of vapour pressure

Schematic: fewer solvent particles at the surface means fewer escape per second.

Solute particles occupy part of the surface, so fewer solvent molecules escape per second: the vapour pressure of a solution of a non-volatile solute is lower than that of the pure solvent.

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