Question

For a solution formed by mixing liquids L and M, the vapour pressure of L plotted against the mole fraction of M in solution is shown in the following figure. Here $x_{L}and{x}_M$ represent mole fractions of L and M, respectively, in the solution. The correct statement(s) applicable to this system is (are)

• A. The point Z represents vapour pressure of pure liquid M and Raoult’s law is obeyed from $x_L=0to{x}_L=1$
• B. Attractive intermolecular interactions between L – L in pure liquid L and M – M in pure liquid M are stronger than those between L – M when mixed in solution
• C. The point Z represents vapour pressure of pure liquid M and Raoult’s law is obeyed when $x_L\to 0$
• D. The point Z represents vapour pressure of pure liquid L and Raoult’s law is obeyed when $x_L\to 1$

Answer 1

Answer: (B), (D)

The correct options are
B Attractive intermolecular interactions between L – L in pure liquid L and M – M in pure liquid M are stronger than those between L – M when mixed in solution
D The point Z represents vapour pressure of pure liquid L and Raoult’s law is obeyed when $x_L\to 1$

The graph shown indicates that there is positive deviation because the observed vapour pressure of L is greater than the ideal pressure
Since the deviation is positive, the intermolecular force between L and M is smaller than the same in pure L and pure M.
Also as $x_L\to 1,x_M\to 0$, the real curve approaches the ideal curve where Raoult’s law will be obeyed.