Question

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 1

Raoult’s law states that for a solution containing a non-volatile solute, the vapor pressure of the solution is directly proportional to mole fraction of the solvent.

P_solution=K_HX_solution

Consider a binary solution of two components A and B (both liquids). Let p total be the total vapor pressure of the solution due to both the components and let p_A and p_B be the partial pressures of component A and B, respectively. Then,

p_A=x_A\(P_{A}^{°} \)

 p_B=x_B\(P_{B}^{°} \)

The total vapor pressure of the solution of liquids A and B is then, by Dalton’s law of partial pressures, the sum of p_A and p_B.

p_Total=p_A+p_B

Substituting for p_A and p_B, we get

p_Total=x_A\(P_{A}^{°} \)+x_B\(P_{B}^{°} \)

The characteristics of the solution which obeys Raoult’s law at all concentrations are as follows: 

(i) In a binary solution of components, A and B, the enthalpy of mixing ΔH_mix is zero, that is, in preparation of an ideal solution no thermal change is observed. 

(ii) In an ideal solution, the volume of mixing (ΔV_mix) is also zero, that is, the final volume of the solution is equal to the sum of volumes of components being mixed