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NEET 2020 · ChemistryPrevious Year Question

The mixture which shows positive deviation from Raoult's law is:

Answer: (C) Ethanol + Acetone. \textbf{Answer:} (C) Ethanol + Acetone \textbf{Solution:} Positive deviation occurs when A-B interactions are \textbf{weaker} than A-A and B-B interactions, raising the vapour pressure above the Raoult's law prediction.

  1. A.Acetone + Chloroform
  2. B.Chloroethane + Bromoethane
  3. C.Ethanol + Acetone✓
  4. D.Benzene + Toluene

Correct Answer

(C) Ethanol + Acetone

Solution & Explanation

\textbf{Answer:} (C) Ethanol + Acetone \textbf{Solution:} Positive deviation occurs when A-B interactions are \textbf{weaker} than A-A and B-B interactions, raising the vapour pressure above the Raoult's law prediction. In Ethanol + Acetone, acetone molecules break the hydrogen bonding present in pure ethanol, weakening the overall interactions → positive deviation. Acetone + Chloroform () is the standard \textbf{negative} deviation pair (H-bonding forms between them). Chloroethane + Bromoethane and Benzene + Toluene behave nearly ideally.

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