Question 1

What is osmosis and how is it different from diffusion?

Accepted Answer

Diffusion is the net movement of ANY particles (solute or solvent) from high to low concentration until uniform. Osmosis specifically involves only the SOLVENT moving through a semipermeable membrane, from the dilute side to the concentrated side. The solute cannot cross the membrane at all — that is the defining difference.

Question 2

What is osmotic pressure?

Accepted Answer

Osmotic pressure (π) is the minimum pressure that must be applied to the SOLUTION side to just stop the net flow of solvent across the semipermeable membrane. It is not a pressure the solution exerts on its own. It is calculated with π = iCRT, where C is molarity, R = 0.0821 L·atm·K⁻¹·mol⁻¹ and T is temperature in kelvin.

Question 3

Why is osmotic pressure preferred for finding the molar mass of macromolecules?

Accepted Answer

For proteins and polymers, even a concentrated solution contains very few moles of solute, so boiling point elevation and freezing point depression are too tiny to measure accurately. Osmotic pressure remains large enough to measure even at low molar concentrations, so π = iCRT (with i = 1 for these non-electrolytes) gives M = wRT/(πV) reliably. This makes osmotic pressure the preferred colligative property for macromolecular molar masses.

Question 4

What is the van't Hoff factor?

Accepted Answer

The van't Hoff factor i is the ratio of the actual number of particles in solution to the number of formula units dissolved. Electrolytes that dissociate have i > 1 (NaCl: 2, CaCl₂: 3, K₂SO₄: 3), non-electrolytes like glucose have i = 1, and associating solutes have i < 1 — benzoic acid dimerises in benzene, giving i ≈ 0.5.

Question 5

Why do colligative properties depend only on the number of particles?

Accepted Answer

All four colligative properties — relative lowering of vapour pressure, boiling point elevation, freezing point depression and osmotic pressure — arise because solute particles reduce the solvent's chemical potential (its tendency to escape or move). This effect scales purely with how many particles are present, not what they are. That is why 0.1 M NaCl behaves roughly like 0.2 M glucose: both give about 0.2 mol of particles per litre.

Question 6

What happens to a red blood cell in a hypotonic solution?

Accepted Answer

A hypotonic solution has a lower solute concentration than the cell's interior, so water flows INTO the cell by osmosis. The red blood cell swells and may burst — this is called haemolysis. In a hypertonic solution the opposite happens: water leaves and the cell shrinks (crenation). A 0.9% NaCl solution is approximately isotonic and causes neither effect.

Osmosis & Colligative Properties — Solvent Moves, the Solute Never Does

Practice

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