Chemistry RELATIVE LOWERING OF VAPOUR PRESSURE
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RELATIVE LOWERING OF VAPOUR PRESSURE

Colligative Properties :

The properties of dilute solutions containing nonvolatile solute, which depends upon relative number of solute and solvent particles but do not depend upon their nature are called colligative properties.

Some of the colligative properties are

(i) Relative lowering of vapour pressure
(ii) Elevation in boiling point
(iii) Depression in freezing point and
(iv) Osmotic pressure.

`text(Factors that Affect the Colligative Property)` : The number of solute particles in solution. To be more accurate, the colligative property depends upon the fraction of solute and solvent particles in solution.

(i) Nature of the solvent
(ii) Independent of the nature of the solute
(iii) Extent of association and dissociation of solute particles in solution.



Relative Lowering of Vapour Pressure :

The mathematical expression for relative lowering of vapour pressure is as follows

`(Delta P)/(P_A^o) = X_B =` mole fraction of solute

`Delta P = P_A^o - P_A =` lowering of vapour pressure

`P_A =` vapour pressure of pure solvent

Molecular mass of non-volatile substance can be determined from relative lowering of vapour pressure

`(P_A^o-P)/(P_A^o) = (W_B//M_B)/(W_A//M_A + W_B//M_B)`

For dilute solution `W_B//M_B < W_A//M_A` and hence `W_B//M_B` may be neglected in the denominator. So

`(P_A^o-P_A)/(P_A^o) = (W_B//M_B)/(W_A//M_A) = W_B/W_A xx M_A/M_B` or `M_B = (W_B//M_A)/W_A(P_A^o/(P_A^o-P_A))`
 
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