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Colligative Properties: Osmotic Pressure

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Colligative Properties: Osmotic Pressure - Lesson Summary

Membranes which allows only solvent particles but not solute particles of solution is calledsemi-permeable membranes (or) SPM. These membranes can be of natural origin (or) synthetic origin.

Vegetable membranes, membranes found under the shell of an egg are examples of natural membranes and cellophane is an example of synthetic membrane.

Thus 'Osmosis' can be defined as the spontaneous flow of solvent through a semi-permeable membrane from a pure solvent to a solution or from a dilute solution to a concentrated solution.

It is important to note that osmosis drives solvent molecules through a semi-permeable membrane from low solute concentrations to high solute concentrations. Osmosis ends when the solute concentration becomes equal on either side of the membrane and equilibrium is attained.

The flow of solvent molecules from low concentration to high concentration can be stopped by applying some extra pressure on the high concentration side. The minimum pressure required to do so is known as the osmotic pressure of the solution.

Thus, osmotic pressure π of a solution is defined as the excess pressure that must be applied to a solution to prevent osmosis from taking place.

Osmotic pressure does not depend on the identity of the solute, but on its concentration.

Osmotic pressure for dilute solutions is proportional to molarity of the solution at a given temperature(T).

                                  π ∝ C (at given T)
π =  C R T
R = Gas constant
C = n2/V
π =  n2RT / V

If W2 grams of solute of molar mass M2 is present in the solution,

n2 = W2/M2

π =  W2RT / M2V

M2 = W2RT / πV

This is widely used to determine the molar masses of polymers and macromolecules, especially biomolecules, as they are generally unstable at higher temperatures and decompose before their boiling point is reached.

If the solutions have the same concentrations (C1 = C2), then π1= π2.

Thus equimolar solutions at the same temperature will have the same osmotic pressure. Such solutions with the same osmotic pressure at a given temperature are called isotonic solutions.

When such solutions are separated by a semi -permeable membrane, no osmosis occurs between them. It is because of this reason that in an intravenous injection of 0.9 per cent mass by volume sodium chloride solution called normal saline solution is used since it is isotonic with the fluid inside the red blood cells.

It is important to note that a high intake of salt in the diet can lead to a higher concentration of fluids in the body tissues because of osmosis. This will result in swelling and puffiness of body parts known as edema.

If a pressure greater than osmotic pressure is applied on a solution, then the solvent will flow from the solution to the pure solvent through the semi-permeable membrane. This process is known as reverse osmosis, is often used for the desalination of sea water for getting fresh drinking water.

Desalination of sea water is carried out using a cellulose acetate semi-permeable membrane placed over a suitable support.


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