Colligative Properties — Part 2: Osmotic pressure and abnormal molar masses: visual lesson, ISC Class 12 Chemistry

Lesson

Osmosis and abnormal molar masses

Part 2 — the fourth colligative property, and what happens when solutes split up or pair up.

In this lesson you will

Compare diffusion and osmosis, and explain osmotic pressure and semipermeable membranes
State the van’t Hoff–Boyle, Charles and Avogadro laws for solutions and derive πV = nRT
Calculate molar mass from osmotic pressure; explain isotonic, hypotonic and hypertonic solutions and reverse osmosis
Use the van’t Hoff factor to find the degree of dissociation or association

Concept

Diffusion and osmosis

Diffusion: spontaneous spreading of solute (and solvent) particles from higher to lower concentration until uniform; no membrane needed. Osmosis: net flow of solvent only through a semipermeable membrane (e.g. parchment, cellophane, copper ferrocyanide film, animal bladder) from pure solvent or a dilute solution into a more concentrated solution.

Table comparing diffusion and osmosis
Feature	Diffusion	Osmosis
Membrane	Not needed	Semipermeable membrane needed
What moves	Solute and solvent	Solvent only
Direction	Solute from high to low concentration	Solvent from dilute to concentrated solution
Stopped by pressure?	No	Yes, by the osmotic pressure

Diagram

Osmotic pressure

At equilibrium the pressure of the extra column of height h equals the osmotic pressure of the solution.

The osmotic pressure $\pi$ of a solution is the excess pressure that must be applied to the solution to just prevent the flow of pure solvent into it through a semipermeable membrane.

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