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Question:

At $35^{\circ} \mathrm{C}$, the vapour pressure of $\mathrm{CS}_{2}$ is $512 \mathrm{~mm}$ $\mathrm{Hg}$ and that of acetone is $344 \mathrm{~mm} \mathrm{Hg} . \mathrm{A}$ solution of $\mathrm{CS}_{2}$ in acetone has a total vapour pressure of $600 \mathrm{~mm} \mathrm{Hg}$. The false statement amongst the following is :

  1. heat must be absorbed in order to produce the solution at $35^{\circ} \mathrm{C}$

  2. Raoult's law is not obeyed by this system

  3. a mixture of $100 \mathrm{~mL} \mathrm{} \mathrm{CS}_{2}$ and $100 \mathrm{~mL}$ acetone has a volume $<200 \mathrm{~mL}$

  4. $\mathrm{CS}_{2}$ and acetone are less attracted to each other than to themselves

Correct Option: , 3

Solution:

The vapour pressure of mixture $(=600 \mathrm{~mm} \mathrm{Hg})$ is greater than the individual vapour pressure of its constituents (Vapour pressure of $\mathrm{CS}_{2}=$ $512 \mathrm{~mm} \mathrm{Hg}$, acetone $=344 \mathrm{~mm} \mathrm{Hg}$. Hence, the solution formed shows positive deviation from Raoult's law.

$\Rightarrow(1) \Delta_{\text {Sol }} H>0$, (2) Raoult's law is not obeyed

(3) $\Delta_{\text {sol. }}$ Volume $>0$

(4) $\mathrm{CS}_{2}$ and Acetone are less attracted to each ether than to themselves.

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