Osmotic Pressure
Colligative Properties & Reverse Osmosis | Solutions Class 12
1. Osmosis and Osmotic Pressure
Osmotic Pressure ($\pi$): The excess pressure that must be applied to the solution side to just stop the flow of solvent molecules through the semi-permeable membrane.
Note: It prevents osmosis; it does not drive it.
2. Mathematical Expression (Van't Hoff Equation)
For dilute solutions, Osmotic Pressure is proportional to the Molarity ($C$) and Temperature ($T$).
Where:
- $\pi$ = Osmotic Pressure (atm or bar)
- $C$ = Molar Concentration ($n/V$ in mol/L)
- $R$ = Gas Constant ($0.0821 \, L \cdot atm \cdot K^{-1} \cdot mol^{-1}$)
- $T$ = Temperature (Kelvin)
- $i$ = Van't Hoff Factor
3. Determination of Molar Mass ($M_2$)
Substituting $C = \frac{n_2}{V} = \frac{w_2}{M_2 \cdot V}$:
Why is this the best method for Polymers/Proteins?
- Measured at room temperature (unlike boiling/freezing points which require heating/cooling).
- Magnitude of $\pi$ is large even for very dilute solutions, making measurement easier and more accurate for macromolecules with high Molar Mass.
4. Types of Solutions
| Type | Condition | Effect on Cell (RBC) |
|---|---|---|
| Isotonic | $\pi_1 = \pi_2$ (Same Conc.) | No change in size. (0.9% NaCl is isotonic with blood). |
| Hypertonic | Solution $\pi >$ Cell $\pi$ | Water flows out. Cell shrinks (Plasmolysis). |
| Hypotonic | Solution $\pi <$ Cell $\pi$ | Water flows in. Cell swells/bursts (Hemolysis). |
5. Reverse Osmosis (RO)
If a pressure larger than the osmotic pressure ($P > \pi$) is applied to the solution side, the direction of solvent flow is reversed.
Application: Desalination of seawater to obtain potable water using cellulose acetate membranes.
Practice Quiz
Test your knowledge on Osmotic Pressure.
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