Question

The vapour pressure of pure liquid A at 300 K is 76.7 kPa and that of pure liquid B is 52.0 kPa.  Assume that A and B form ideal solutions and gaseous mixtures. Consider the equilibrium composition of an ideal solution of A and B in which the total pressure above the solution is 70.5 kPa.  

(a) Determine the mole fractions of A and B in the liquid solution and in the vapour phase above the solution.  

(b) If the actual vapor pressures of A and B are 60.0 kPa and 18.1 kPa respectively, comment on the strength of intermolecular forces in the solution versus the pure liquids.

(c) If the vapor above the solution is condensed and allow to reach a new equilibrium between liquid and vapor phases, determine the mole fractions of both A and B in the new vapour phase.

Answer 1

b) as we know that vapour pressure is caused by number of molecules getting evaporated from the surface hence more number of evaporating molecules higher the vapour pressure and it also indicates the compund's intermolecular forces.

Now in the pure phase both the liquids have quite stronger intermolecular forces as they have lower vapour pressure as they require more energy to break down the intermolecular forces and allow the molecules to evaporate while when in a solution they have higher vapor pressure indicating they now have weaker intermolecular forces and therefore more molecules are getting evaporated causing higher vapor pressure.