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 when $x_L\to 0$
• 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 $L$ and Raoult's law is obeyed when $x_L\to 1$.
• D. The point $Z$ represents vapour pressure of pure liquid $M$ and Raoult's law is obeyed from $x_L=0$ to $x_L=1$

Answer 1

Answer: (B), (C)

We know for Raoult's law
$P=P_L^0{x}_L+P_M^0{x}_M$
Vapour pressure of $L=P_L=P_L^0{x}_L$
$=P_L^0\left(1-x_M\right),$ at $x_m\to 0$ or $x_L\to 1$
$P_L=P_L^0$ which follow the graph shown at point $Z$
Line $YZ$ forms tangent at a point $Z$ which tells that it follows Raoult's law.
$\therefore$ option D is correct.
Let point $P$' show vapor pressure of mixture and point $P$'' shows vapor pressure of the ideal mixture.
Since $P^{\prime }>P^{\prime \prime }$, it tells that in mixture $L-M$ bonds break easily than that in the pure state as the vapour pressure of the mixture is more than it should be more in an ideal state.