Osmotic PressureJEE Main
π = iCRT — solutions that push — interactive Chemistry simulation for IIT-JEE.
Concept
Separate solution from pure solvent with a semipermeable membrane and solvent flows into the solution (osmosis). The excess pressure that just stops the flow is the osmotic pressure — astonishingly large: a 0.1 M solution can hold up a 25-metre water column. It obeys a perfect gas-lookalike law, π = iCRT.
Key formula
Derivation
Thermodynamically, dissolved solute lowers the solvent's chemical potential; pressure raises it back. Balancing the two gives van 't Hoff's dilute-solution result — formally identical to the ideal gas law with C = n/V.
Being proportional to particle COUNT, it's colligative — the i factor rides along for electrolytes. It's the most sensitive colligative property: best for macromolecule molar masses (proteins, polymers).
Scenarios to explore
- Osmotic Pressure — π = iCRT — U-tube osmosis and isotonic solutions.
Real-world applications
- Reverse osmosis desalination (apply P > π).
- IV drips must be isotonic (~0.9% saline) or cells burst/shrivel.
- Trees lift sap partly by osmotic gradients.
JEE exam tips
- Molar-mass determination: M = wRT/πV — best for polymers because π stays measurably large.
- Isotonic condition: i₁C₁ = i₂C₂.
- 0.1 M at 300 K → π ≈ 2.46 atm: memorise the scale.
Common mistakes
- Using molality — π needs molarity (volume-based).
- Dropping i for salts.
- Isotonic ⇒ equal CONCENTRATION only if both solutes have the same i.
Exam traps to avoid
- Hypertonic bathing solution SHRINKS cells (water leaves).
- Reverse osmosis needs pressure EXCEEDING π on the solution side.