Question

Consider the following statements about the solutions formed by mixing two liquids.

• [A.] An ideal solution thus formed obeys Raoult's law throughout the composition range.

• [B.] Mixture of chloroform and acetone shows negative deviation from Raoult's law.

• [C.] Mixture of aniline and phenol shows positive deviation from Raoult's law.
Select

• A. A and C only
• B. A and B only
• C. B and C only
• D. A only

Hint:

Whenever mixing two components results in the formation of new, stronger bonds (like hydrogen bonding between chloroform-acetone or phenol-aniline), the molecules are held tightly in the liquid phase. This always results in a lower vapor pressure, meaning a negative deviation from Raoult's Law.

Answer 1

The Correct Option is B

Concept: Binary solutions composed of volatile liquids are categorized into ideal and non-ideal solutions based on how they interact and whether they adhere to Raoult's Law:

• Ideal Solutions: Obeys Raoult's law precisely at all temperatures and across the entire range of concentrations. For an ideal mixture of components $A$ and $B$, the intermolecular forces between the different molecules ($A-B$ interactions) are exactly equal in magnitude to the pure component interactions ($A-A$ and $B-B$ interactions). Consequently, \(\Delta H_{\text{mixing}} = 0\) and \(\Delta V_{\text{mixing}} = 0\).

• Non-Ideal Solutions with Negative Deviation: Occurs when the new intermolecular attractive forces between unequal components ($A-B$) are significantly stronger than the cohesive forces present in the isolated pure liquids ($A-A$ and $B-B$). This stronger binding holds molecules more tightly in the liquid phase, decreasing their tendency to escape into the vapor state. As a result, the total vapor pressure of the solution is lower than predicted by Raoult's law. Here, \(\Delta H_{\text{mixing}} \lt 0\) and \(\Delta V_{\text{mixing}} \lt 0\).

• Non-Ideal Solutions with Positive Deviation: Occurs when the adhesive interactions between the components ($A-B$) are weaker than the pure component self-interactions ($A-A$ and $B-B$). This makes it easier for molecules to break away into the gas phase, raising the vapor pressure above the theoretical Raoult's law curve. Here, \(\Delta H_{\text{mixing}} \gt 0\) and \(\Delta V_{\text{mixing}} \gt 0\).
Let us evaluate each statement carefully to determine its accuracy:

Step 1: Evaluation of Statement A.
"An ideal solution thus formed obeys Raoult's law throughout the composition range."
By standard thermodynamic definition, a solution is classified as ideal if and only if the partial vapor pressure of each volatile component in the mixture is directly proportional to its mole fraction at all concentrations and temperatures. Thus, statement A is completely correct.

Step 2: Evaluation of Statement B.
"Mixture of chloroform (\(\text{CHCl}_3\)) and acetone (\(\text{CH}_3\text{COCH}_3\)) shows negative deviation from Raoult's law."
In pure acetone, molecules are held together by ordinary dipole-dipole interactions. Similarly, pure chloroform molecules experience weak dipole-dipole interactions. However, when chloroform and acetone are mixed together, a strong intermolecular hydrogen bond forms between the highly polarized hydrogen atom of chloroform and the electronegative oxygen atom of the acetone carbonyl group: \[ \text{(CH}_3)_2\text{C}=\text{O} \cdots \text{H}-\text{CCl}_3 \] Because these newly formed cross-interactions ($A-B$) are stronger than the original interactions ($A-A$ and $B-B$), the escaping tendency of both molecules decreases, dropping the vapor pressure below the ideal threshold. This constitutes a negative deviation. Thus, statement B is completely correct.

Step 3: Evaluation of Statement C.
"Mixture of aniline and phenol shows positive deviation from Raoult's law."
Phenol (\(\text{C}_6\text{H}_5\text{OH}\)) contains an acidic hydroxyl hydrogen, while aniline (\(\text{C}_6\text{H}_5\text{NH}_2\)) contains a basic lone pair on its nitrogen atom. When aniline and phenol are blended together, the intermolecular hydrogen bonding between the phenolic proton and the nitrogen lone pair of aniline is significantly stronger than the self-hydrogen bonding present in pure phenol or pure aniline. Because the $A-B$ intermolecular forces are stronger than the $A-A$ and $B-B$ forces, this mixture exhibits a negative deviation from Raoult's law, not a positive deviation. Therefore, statement C is incorrect. Combining our individual assessments, statements A and B are correct, while statement C is false. This corresponds directly to option (2).