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

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

P_A = P^o_A × X_A

P_B = P⁰_{B ×} X_B

P = P_A + P_B = P^o_A × X_A + P⁰_{B ×} X_B

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,

X_{A +} X_B = 1

X_A = 1- X_B

P = (1-X_B)P⁰_A + X_BP⁰_B

P = P⁰_A + X_B(P_B⁰ – P⁰_A)

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.