Question

At ${35}^{\circ }C$, the vapour pressure of $C{S}_2$ is 512 mm Hg and that of acetone is 344 mm Hg. A solution of $C{S}_2$ in acetone has a total vapour pressure of 600 mm Hg. The false statement amongst the following is:

• A. A mixture of 100 mL $C{S}_2$ and 100 mL acetone has a volume < 200 mL
• B. Heat must be absorbed in order to produce the solution at ${35}^{\circ }C$
• C. Raoult’s law is not obeyed by this system
• D. $C{S}_2$ and acetone are less attracted to each other than to themselves.

Answer 1

The correct option is A A mixture of 100 mL $C{S}_2$ and 100 mL acetone has a volume < 200 mL

$P_{\text{Total}}=P_T=P_A^{\circ }{X}_A+P_B^{\circ }{X}_B$

The maximum value $X_A$ can hold is $1$, and hence the maximum value of $P_T$ should come out to be $512\text{mm}$ of Hg, which is less than the value of $P_T$ observed ($600\text{mm}$ of Hg). Therefore, positive deviation from Raoult’s law is observed. This implies that A-A interactions and B-B interactions are stronger than A-B interactions. As we know, for a system not obeying Raoult’s law and showing positive deviation, $\Delta V_{\text{mix}}>0,\Delta H_{\text{mix}}>0$.