Question

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

• A. The point $\text{Z}$ represents vapour pressure of pure liquid $\text{M}$ and Raoult’s law is obeyed from $X_{L}0$ to $X_L=1$
• B. The point $\text{Z}$ represents vapour pressure of pure liquid $\text{L}$ and Raoult’s law is obeyed when $x_{\to }L1$
• C. The point $\text{Z}$ represents vapour pressure of pure liquid $\text{M}$ and Raoult’s law is obeyed when $x_L\to 0$
• D. Attractive intermolecular interactions between $\text{L-L}$ in pure liquid $\text{L}$ and $\text{M-M}$ in pure liquid $\text{M}$ are stronger than those between $\text{L-M}$ when mixed in solution.

Answer 1

Answer: (B), (D)

The correct options are
B The point $\text{Z}$ represents vapour pressure of pure liquid $\text{L}$ and Raoult’s law is obeyed when $x_{\to }L1$
D Attractive intermolecular interactions between $\text{L-L}$ in pure liquid $\text{L}$ and $\text{M-M}$ in pure liquid $\text{M}$ are stronger than those between $\text{L-M}$ when mixed in solution.

• The point Z represents the vapour pressure of a pure liquid M but Raoult's law is not obeyed from $X_L=0to{X}_L=1$ because the curve is not straight line
• The point $\text{Z}$ represents the vapour pressure of a pure liquid $\text{L}$ as the mole fraction of M is 0 and the total vapour pressure of the solution is the same as the vapour pressure of the pure liquid $\text{L}$. When $x_L\to 1$, we can see the curve is taking the shape of a straight line, hence constant slope
• While a $x_L\to 0$ the curve of the slope is not constant i.e. the shape of the curve appears to be parabolic so Raoult's law is not obeyed
• If we connect the point 1 and 2 by a straight line, we will observe that the straight line will always remain below the curve as it shows a positive deviation from Raoult’s law. Attractive intermolecular interactions between $\text{L-L}$ in pure liquid $\text{L}$ and $\text{M-M}$ in pure liquid $\text{M}$ are stronger than those between $\text{L-M}$ when mixed in solution.