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Q.A solution of acetone in ethanol : (A) obeys Raoult's law. (B) forms an ideal solution. (C) shows a positive deviation from Raoult's law. (D) shows a negative deviation from Raoult's law.

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Acetone–ethanol mixtures form hydrogen bonds weaker than those in pure ethanol, making escape easier and raising vapor pressure above the ideal prediction. The solution shows a positive deviation from Raoult's law — option (C).

Types of Solutions: Understanding Deviations from Ideality

Raoult's law predicts that the partial vapor pressure of each component in a solution is proportional to its mole fraction: Pi=xiPi0P_i = x_i P_i^0. This holds perfectly only for ideal solutions, where the intermolecular forces between unlike molecules (A–B) are identical in strength to those between like molecules (A–A and B–B). When these forces differ, the solution deviates from Raoult's law.

Positive deviation occurs when A–B interactions are weaker than the average of A–A and B–B interactions. Molecules escape more easily into the vapor phase, so the total vapor pressure exceeds the ideal prediction.

Negative deviation occurs when A–B interactions are stronger than those in the pure components. Molecules are held more tightly in the liquid, reducing vapor pressure below the ideal value.

The key is to compare the intermolecular forces in the mixture with those in the pure liquids.


Step-by-Step Analysis

  1. Identify the intermolecular forces in pure ethanol

    Ethanol (CX2HX5OH\ce{C2H5OH}) has a hydroxyl group that forms strong hydrogen bonds between molecules. These O–H···O interactions are quite robust, holding ethanol molecules together tightly in the liquid phase.

  2. Identify the intermolecular forces in pure acetone

    Acetone (CHX3COCHX3\ce{CH3COCH3}) is a polar molecule with a carbonyl group, but it cannot donate hydrogen bonds (no O–H or N–H). It can only accept hydrogen bonds. In pure acetone, the dominant forces are dipole–dipole interactions, which are weaker than hydrogen bonds.

  3. Examine the acetone–ethanol interaction

    When acetone and ethanol mix, ethanol can donate a hydrogen bond to the oxygen of acetone's carbonyl group, forming an O–H···O=C interaction. However, this hydrogen bond is weaker than the O–H···O–H hydrogen bonds present in pure ethanol, because the carbonyl oxygen is a less effective hydrogen-bond acceptor than the hydroxyl oxygen.

  4. Compare net intermolecular forces

    • In pure ethanol: strong H-bonds (O–H···O–H)
    • In the mixture: weaker H-bonds (O–H···O=C) plus dipole–dipole forces …

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