Class 12 Chemistry


Osmosis and Osmotic Pressure

Semi-permeable Membrane: Some membranes appear to be continuous but they have minute pores through which small molecules can pass. Such a membrane is called semi-permeable membrane.

Osmosis: When a semi-permeable membrane is placed between a solvent and solution, the solvent molecules flow through the membrane from pure solvent to solution. This process of flow of solvent is called osmosis.

Osmotic Pressure: The flow of solvent from solvent side to solution side (across a semi-permeable membrane) can be stopped by applying some extra pressure. The pressure which is just enough to stop osmosis is called osmotic pressure of the solution.

Fore dilute solutions, osmotic pressure is proportional to the molarity (C) of the solution at a given temperature.

`Π =text(CRT)`

Here, Π is osmotic pressure and R is the gas constant.

Now, `Π=((n_2)/V)RT`

Here, V is volume of solution in litres containing n2 moles of solute. If w2 g of solute, of molar mass M2 is present in solution then `n_2=(w_2)/(M_2)`

So, `ΠV=(w_2R\T)/(M_2)`

Or, `M_2=(w_2R\T)/(ΠV)`

Significance of Osmotic Pressure: Osmotic pressure can be utilized to determine molar masses of solutes. This method has advantage over other methods because pressure measurement is around the room temperature and molarity is used instead of molality. Compared to other colligative properties, its magnitude is large even for very dilute solutions. This method of determining molar mass of solutes is especially useful for biomolecules because biomolecules are not stable athigher temperatures and polymers have poor solubility.

Isotonic Solution: Two solutions with same osmotic pressure at a given temperature are called isotonic solutions. No osmosis happens when two isotonic solutions are separated by a semi-permeable membrane.

Hypertonic Solution: A solution with higher osmotic pressure compared to another solution is called hypertonic solution with respect to the second solution. In this case, osmosis happens from second solution to the first solution.

Hypotonic Solution: A solution with lower osmotic pressure compared to another solution is called hypotonic solution with respect to the second solution. In this case, osmosis happens from first solution to the second solution.

Example: 200 cm3 of an aqueous solution of a protein contians 1.26 g of the protein. The osmotic pressure of such a solution at 300 K is found to be 2.57 `xx` 10-3 bar. Calculate the molar mass of the protein.

Answer: Given: Π = 2.57 `xx` 10-3 bar

V = 200 cm3 = 0.200 litre

T = 300 K

R = 0.083 L bar mol-1 K-1

Now, molarity can be calculated as follows:



`=61.022` g mol-1

Reverse Osmosis: The direction of osmosis can be reverse by applying a pressure larger than the osmotic pressure to the solution side. This phenomenon is called reverse osmosis.