OSMOSIS AND OSMOTIC PRESSURE

COLLIGATIVE PROPERTIES (OSMOSIS AND OSMOTIC PRESSURE)

(Latin : co means together, ligare means to bind)

The properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

There are four colligative properties :

1.      Relative lowering of vapour pressure

2.      Elevation of boiling point

3.      Depression of freezing point

Osmotic pressure.

OSMOSIS

The phenomenon of osmosis was first studied by Abbe Nollet in 1748.

Osmosis is the process of the flow of solvent through a semi-permeable membrane from pure solvent to the solution.

(Semi-permeable membrane is the membrane which allows only the solvent and not the solute to pass through it)

It is different from diffusion, in diffusion solute and solvent both can move from the region of their higher concentration to lower concentration and in osmosis, solvent moves from lower concentration to the higher concentration.

For example, raw mangoes shrivel when pickled in brine solution, wilted flowers revive when placed in fresh water, blood cells collapse when suspended in saline water, etc.

If you place a semi permeable membrane between pure solvent and solution, the solvent molecules move from the chamber of pure solvent to the chamber of solution, and this process is termed as osmosis. The movement of solvent will continue till the equilibrium is attained. This movement or flow of the solvent can e stopped if some extra pressure is applied on the solution.

This process which stops the flow of solvent through semi permeable membrane is called osmotic pressure.

In other words, the osmotic pressure of a solution is the excess pressure that must be applied to a solution to prevent osmosis (to stop the flow of solvent molecules)

For dilute solutions, it has been found experimentally that osmotic pressure is proportional to the molarity, C of the solution at a given temperature T.

Thus    π = C R T,

π  is the osmotic pressure, R is the gas constant.

π = (n₂/V) R T

Where V is the volume of the solution in litres n₂ is the number of moles of solute. If w₂ and M₂ are the given mass and molar mass of the solute, then n₂ = w₂ / M₂.

V π  = (w₂/M₂) R T.

Advantages of osmotic pressure method for calculating the molar mass of soute

1.      The process can be done at room temperature

2.      And the molarity term is used where in other methods molality is used.

Isotonic solution: The solutions which have same osmotic pressure at the same temperature are called isotonic solution. The osmotic pressure is same because the solutions have same concentration.

Hypotonic and Hypertonic solution: The solution which has low osmotic pressure is known as hypotonic solution and the solution which has high osmotic pressure is known as hypertonic solution at the same temperature are called isotonic solution.

The concentration of hypotonic solution is lesser than the concentration of hypertonic solution.

The solvent moves from hypotonic solution to hypertonic solution till they become isotonic solution.

Edema -  People taking  a lot of salt or salty food experience water retention in tissue cells and intercellular spaces because of osmosis. The swelling or puffiness is called edema.

Reverse Osmosis: The process of movement of solvent through the semi permeable membrane from solution to the pure of solvent by applying excess pressure on solution side is called reverse osmosis.

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