Q. 74.2( 18 Votes )

State Raoult’s law for a solution containing volatile components. Write two characteristics of the solution which obeys Raoult’s law at all concentrations.

Answer :

Raoult’s law for volatile solutes states that in a solution, the vapour pressure of a component at a given temperature is equal to the mole fraction of that component in the solution multiplied by the vapour pressure of that component in the pure state.

Mathematically:


The vapour pressure P of a solution containing two components A and B is



PA = PoA × XA


PB = P0B × XB


P = PA + PB = PoA × XA + P0B × XB


where, is the vapour pressure of component A


is the vapour pressure of component B


is the vapour pressure of component A in its pure state


is the vapour pressure of component B in its pure state


is the mole fraction of component A


is the mole fraction of component B


Now we know that,


XA + XB = 1


XA = 1- XB


P = (1-XB)P0A + XBP0B


P = P0A + XB(PB0 – P0A)


For a solution to be able to obey Raoult’s law at all concentrations, it should be an ideal solution. For a solution to be called ideal, it should have the following two characteristics:


The enthalpy of mixing of the pure components to form the solution should be zero, that is, . It means that no heat is absorbed or released during mixing.


The volume of mixing the pure components should be zero, that is, . This means that the volume of the solution would be equal to the sum of volumes of the two components.


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