RAOULT’S LAW

François-Marie Raoult (10 May 1830 – 1 April 1901) . He was a French chemist. He gave a law for binary solutions , which is called  Raoult’s law.

DEFINITION

This law states that for a binary solution, the partial vapor pressure of each component is directly proportional to its mole fraction at a given temperature. E.g. consider a binary solution of two volatile components A and B.

Here _PA^° and PB^°  are constants and are called vapor pressures of components in the pure state. The total vapor pressure of solution is given by the following equation: 

P_S = P_A  +  P_B

This relationship is called Raoult’s law.

EXPLANATION OF RAOULT’S LAW

IDEAL SOLUTIONS

 

Q: - What are ideal solutions?

Ans: - (1) An ideal solution is that solution which obeys Raoult’s law  .

     i.e.    P_S  = P_A + P_B              

(2) ∆Hmixing = 0

(3) ∆Vmixing = 0

(4) The strength of A---B interactions is same as that of A---A and B----B interactions.

(5) E.g.  (i)  n-hexane + n-heptane.

                (ii) Benzene + Toluene. 

DIFFERENCE BETWEEN IDEAL AND NON-IDEAL SOLUTIONS

 

IDEAL SOLUTIONS	NON IDEAL SOLUTIONS
(1)They obey Raoult’s law.	(1)They do not obey Raoult’s law.
(2)∆Hmixing = 0.	(2) ∆Hmixing ≠ 0
(3)  ∆Vmixing = 0.	(3) ∆Vmixing ≠0.
(4) The strength of A---B interactions is same as that of A---A and B----B interactions.	(4) The strength of A---B interactions is not same as that of A---A and B----B interactions.
(5)E.g. (i) n-hexane + n-heptane.               (ii) Benzene + Toluene.	(5) E.g. (i) Benzene + acetone.               (ii) Acetone + chloroform.

CONCEPTUAL QUESTION

Q:- Consider two solutions:  one is formed by dissolving 245 g of glucose in 1 kg of water and  another is formed by adding 245 g of sucrose in 1 kg of water. Which one will have higher vapor pressure over the solution? Account for your answer.

Ans: - one with lesser number of moles of non-volatile solute.

 

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