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Saturday, April 25, 2020

What is the Raoult's law and it's application ?

(1) Vapour pressure of a number of binary solutions of volatile liquids such as benzene and toluene at constant temperature gave the following generalization which is known as the Raoult's law.
(2) Raoult's law states that "The partial pressure of any volatile component of a solution at any temperature is equal to the vapour pressure of the pure component multiplied by the mole fraction of that component in the solution
(A) Vapour pressure of liquid-liquid Solution:
(3) Suppose a binary solution contains nA moles of a volatile liquid A and nB moles of a volatile liquid B, if PAand PB are partial pressure of the two liquid components, the according to Raoult's law
(4) If the vapour behaves like an ideal gas, thenaccording to Dalton's law of partial pressures, the total pressure P is given by 
Graphical representation of Raoult's law:
(5) The relationship between vapour pressure and mole fraction of an ideal solution at constant temperature is shown. The dashed lines 1 and 2 represent the partial pressure of the components. The total vapour pressure is given by 3rd line in the above figure.

(B) Vapour pressure of Solid-liquid Solution:
(1) Vapor pressure, when a small amount of a non-volatile solute (solid) is added to the liquid (solvent). It is found that the vapour pressure of the solution is less than that of the pure solvent.
(2) The lowering of vapour pressure is due to the fact that the solute particles occupy a certain surface area and evaporation takes place from the surface only. and
(3) The particles of the solvent will have a less tendency to change into vapour i.e. the vapour pressure of the solution will be less than that of the pure solvent and it is termed as lowering of vapour pressure.
For a solution of non-volatile solute with volatile solvent.

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