What is Osmotic Pressure? (Class 12 Chemistry)

Before defining the pressure, you must understand the process.

• Osmosis is the natural movement of pure solvent molecules (like water) through a Semipermeable Membrane (SPM) from a region of lower solute concentration (pure water) into a region of higher solute concentration (salty water). The water tries to dilute the salt.

Imagine a U-tube with pure water on the left, salty water on the right, and an SPM in the middle. The pure water will naturally rush to the right side, raising the water level on the right.

Definition: The extra, external physical pressure that must be applied to the solution side to just stop the natural flow of solvent molecules (osmosis) across the semipermeable membrane is called Osmotic Pressure ($\pi$).

The osmotic pressure of a dilute solution is directly proportional to its molar concentration (C) and temperature (T). The formula is: $\pi = C R T$

• $\pi$ (Pi) = Osmotic Pressure.
• C = Concentration (Molarity) of the solution.
• R = Universal Gas Constant.
• T = Absolute Temperature (in Kelvin). (Note: If the solute dissociates into ions, like NaCl, you must multiply by the Van't Hoff factor 'i').