Question

At 35$^{\circ}$C, the vapour pressure of $CS_2$ is 512 mm Hg and that of acetone is 344 mm Hg. A solution of $CS_2$, in acetone has a total vapour pressure of 600 mm Hg. The false statement amongst the following is :

• A. heat must be absorbed in order to produce the solution at 35$^{\circ}$C
• B. a mixture of 100 mL $CS_2$, and 100 mL acetone has a volume < 200 mL
• C. $CS_2$, and acetone are less attracted to each other than to themselves
• D. Raoult's law is not obeyed by this system

Answer 1

The Correct Option is B

 To solve this question, we need to analyze the information given and understand the concept of vapor pressure in relation to solutions:

1. The vapor pressure of pure \(CS_2\) is 512 mm Hg and that of pure acetone is 344 mm Hg.
2. A solution of \(CS_2\) in acetone has a total vapor pressure of 600 mm Hg.

Let's examine each statement given in the options to identify the false one:

1. Option 1: Heat must be absorbed in order to produce the solution at 35\(^{\circ}\)C
   • This implies an endothermic process. Since the vapor pressure of the solution is higher than both the individual components, it could result from reduced intermolecular attraction between the two components, suggesting an endothermic process. Thus, this statement can be considered true.
2. Option 2: A mixture of 100 mL \(CS_2\) and 100 mL acetone has a volume < 200 mL
   • Upon mixing, most liquids do not occupy a volume that is exactly additive because of molecular interactions. However, this behavior would not necessarily indicate a false statement. Mixing could lead to lesser or greater volumes, so this statement might not always be true and can be considered false.
3. Option 3: \(CS_2\) and acetone are less attracted to each other than to themselves
   • This is plausible because the total vapor pressure of the solution is higher than the vapor pressures of both components. This suggests weaker interactions between \(CS_2\) and acetone, causing easier vaporization from the solution.
4. Option 4: Raoult's law is not obeyed by this system
   • Since the vapor pressure is higher than what would be expected by ideal behavior, this system likely deviates from Raoult's Law, which is applicable to ideal mixtures.

Conclusion: The false statement is: a mixture of 100 mL \(CS_2\) and 100 mL acetone has a volume < 200 mL. This statement is based on an assumption regarding volume contraction, which is not necessarily a rule, making it false in the context of this question.