a colligative property as it depends on the number of solute molecules and not on their identity. 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: P = C R T ( . ) Here P is the osmotic pressure and R is the gas constant.
P = ( n / V ) R T ( . ) Here V is volume of a solution in litres containing n moles of solute. If w grams of solute, of molar mass, M is present in the solution, then n = w / M and we can write, P V = R T M ( . ) or M = ∏ R T V ( .
) Thus, knowing the quantities w , T, P and V we can calculate the molar mass of the solute. Measurement of osmotic pressure provides another method of determining molar masses of solutes. This method is widely used to determine molar masses of proteins, polymers and other Fig. .
: The excess pressure equal to the osmotic pressure must be applied on the solution side to prevent osmosis. macromolecules. The osmotic pressure method has the advantage over other methods as pressure measurement is around the room temperature and the molarity of the solution is used instead of molality. As compared to other colligative properties, its magnitude is large even for very dilute solutions.
The technique of osmotic pressure for determination of molar mass of solutes is particularly useful for biomolecules as they are generally not stable at higher temperatures and polymers have poor solubility. Two solutions having same osmotic pressure at a given temperature are called isotonic solutions . When such solutions are separated by semipermeable membrane no osmosis occurs between them. For example, the osmotic pressure associated with the fluid inside the blood cell is equivalent to that of .
% (mass/volume) sodium chloride solution, called normal saline solution and it is safe to inject intravenously. On the other hand, if we place the cells in a