Question

Which one of the following graphs accurately represents the plot of partial pressure of CS₂ vs its mole fraction in a mixture of acetone and CS₂ at constant temperature?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Positive deviation is often associated with $\Delta H_{mix}>0$ (endothermic) and $\Delta V_{mix}>0$ (expansion upon mixing).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A mixture of acetone and carbon disulfide ($CS_2$) is a classic example of a non-ideal solution showing positive deviation from Raoult's law. This occurs because the intermolecular attractions between acetone-$CS_2$ are weaker than acetone-acetone or $CS_2$-$CS_2$ attractions.

Step 2: Detailed Explanation:
In a solution with positive deviation:
- The vapor pressure of each component is higher than predicted by Raoult's law.
- The graph of partial pressure ($P_{CS2}$) against mole fraction ($x_{CS2}$) will not be a straight line but a curve bowed upwards.
- This is because the molecules find it easier to escape into the vapor phase due to weaker $A-B$ interactions.
Step 3: Final Answer:
The graph accurately representing the plot shows positive deviation (curved upward).