Apparatus Diagram
Labelled Components and Explanation
| Component | Description |
|---|---|
| Semipermeable membrane | Allows only solvent (water) molecules to pass; blocks solute particles. Examples: pig bladder, cellophane, cellulose acetate. |
| Solution side | Higher solute concentration → higher osmotic pressure → lower water activity |
| Pure solvent side | Pure water → zero solute → lower osmotic pressure → higher water activity |
| Osmotic flow direction | Solvent flows from pure solvent (lower π) through membrane to solution (higher π) |
| Rise in liquid level | The hydrostatic pressure at equilibrium balances the osmotic pressure |
| Osmotic pressure (π) | The excess pressure that must be applied to solution side to STOP osmotic flow |
Reverse Osmosis Setup
Reverse Osmosis Semi-permeable membrane Salt Solution (high concentration) Pure Water (permeate) P_ext > π $H_{2}O$ Solute ions ($Na^{+}$, $Cl^{-}$) — blocked by membrane Water molecules — pass through membraneIn reverse osmosis, applied pressure P_ext > π_osmotic forces solvent from the solution side through the membrane — the direction is OPPOSITE to natural osmosis. Applications: desalination of sea water (removing NaCl), water purification in homes (RO filters).
Key Measurements
- Osmometer measures osmotic pressure
- π = iCRT (C in mol/L, T in K)
- For 0.9% NaCl (isotonic saline): π ≈ 7.7 atm at 37°C — equal to blood plasma