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 $x _{ L } \rightarrow 0$
• 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 $x _{ L } \rightarrow 1$.
• 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: (C)

The point $Z$ represents vapour pressure of pure liquid $L$ and Raoult's law is obeyed when $x _{ L } \rightarrow 1$.

Answer 2

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 $x _{ m } \rightarrow 0$ or $x _{ L } \rightarrow 1$
$P_{L}=P_{L}^{0}$ which follow the graph shown at point $Z$
Line $Y Z$ 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 $P ' > P ''$, 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.