Solutions — Part 2: Colligative properties and the van’t Hoff factor: visual lesson, CBSE Class 12 Chemistry

Lesson

Colligative properties

Part 2 — four properties that depend only on how many solute particles are present, not on what they are.

In this lesson you will

Explain the relative lowering of vapour pressure and use it to find molar mass
Calculate the elevation of boiling point and the depression of freezing point
Explain osmosis, osmotic pressure, isotonic solutions and reverse osmosis, and use osmotic pressure to find molar mass
Use the van’t Hoff factor to correct for dissociation and association and to find the degree of dissociation or association

Concept

What “colligative” means

Colligative means “tied together”: the four effects all come from 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.

Table showing that a mole of glucose or urea gives one unit of effect, while a fully dissociated mole of sodium chloride gives about two and calcium chloride about three
Solution in 1 kg of water	Particles of solute	Size of effect
1 mol glucose	1 mol of molecules	1 unit
1 mol urea	1 mol of molecules	1 unit
1 mol sodium chloride (fully split)	2 mol of ions	about 2 units
1 mol calcium chloride (fully split)	3 mol of ions	about 3 units

Diagram

Lowering of vapour pressure

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

Solute particles take up 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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