Osmotic Pressure : Flow based on concentration difference

Define Osmotic Pressure?

Osmotic pressure is the pressure that is applied to stop the flow of solvent molecules from a dilute solution to a concentrated solution through a semi-permeable membrane. Now the above statement has many terms that need to be explained. Let us consider the semi permeable membrane; it is a membrane that selectively allows the movement of particles through it. At some point or the other, we all have seen shriveling of raw mango when it is pickled in salt water. Blood cells are known to collapse when exposed to saline water. What causes these phenomena ? It has been observed that when a dilute solution is separated from a concentrated solution of the same type by a membrane, the solvent molecules flow from dilute solution to the concentrated solutions until equilibrium is established. This process of flow of solvent molecules due to the difference in concentration is known as osmosis.

The flow of solvent molecules through the membrane can be stopped by applying external pressure. As we discussed at the beginning, the pressure that just stops the flow of solvent particles is known as the osmotic pressure. It has been seen that osmotic pressure is a colligative property as it only depends on the concentration of the solution. In other terms, osmotic pressure is the extra pressure that must be applied in order to prevent osmosis.  Experimental data shows that this pressure is directly proportional to the molarity (number of moles of solute per litre of solution) of the solution at a certain given temperature.




Π=osmotic pressure

C=molarity of the solution

R=Universal Gas constant:


Since molarity is the number of moles of solute per litre of the solution . Suppose w2 is the mass of solute having molar mass M2 in a solution of volume V, then we can write,

\( \pi = \frac{w_2 RT}{M_2 V} \)

The above equation is extensively used in the determination of molar mass of complex molecules of proteins, polymers and other macromolecules. This method is preferably used for bio-molecules as it shows a significant value even at room temperature.Learn more about osmosis at BYJU’s.’

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