Question

Which of the following solutions shows positive deviation from Raoult's law?

• A. Acetone + Aniline
• B. Acetone + Ethanol
• C. Water + Nitric acid
• D. Chloroform + Benzene

Hint:

Positive deviation occurs when mixing causes weakening of intermolecular attractions. \[ \text{Weaker attractions} \Rightarrow \text{Higher vapor pressure} \] Common examples:
• Ethanol + Acetone
• Ethanol + Cyclohexane

Answer 1

The Correct Option is B

Concept: A solution shows:
• Positive deviation from Raoult's law when intermolecular forces between unlike molecules are weaker than those between like molecules.
• Negative deviation when intermolecular attractions between unlike molecules are stronger. For positive deviation: \[ A-B < A-A \text{ and } B-B \] As a result:
• Vapor pressure increases
• Boiling point decreases

Step 1: Analyzing Acetone + Aniline.
Acetone and aniline interact strongly through hydrogen bonding between: \[ \text{C=O of acetone and } -NH_2 \text{ of aniline} \] Thus, intermolecular attraction becomes stronger. Therefore, this mixture shows negative deviation.

Step 2: Analyzing Acetone + Ethanol.
Pure ethanol molecules are strongly associated through hydrogen bonding. When acetone is added, the strong ethanol-ethanol hydrogen bonding gets weakened. The new acetone-ethanol interactions are comparatively weaker. Hence: \[ A-B < A-A \] Thus, vapor pressure increases above ideal value. Therefore, the mixture shows: \[ Positive deviation from Raoult's law \]

Step 3: Analyzing remaining options.

• Water + Nitric acid shows strong ion-dipole interactions and hydrogen bonding. Hence, it shows negative deviation.
• Chloroform + Benzene forms stronger intermolecular interactions than ideal case. Hence, it also shows negative deviation. Thus, only Acetone + Ethanol shows positive deviation.