Question:

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

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For negative deviation, intermolecular attractions between the solute and solvent must be stronger than those between solvent molecules.
Updated On: May 3, 2025
  • Benzene-acetone
  • Benzene-chloroform
  • Benzene-ethanol
  • Benzene-carbon tetrachloride
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The Correct Option is B

Approach Solution - 1

Negative deviation from Raoult's law occurs when the interactions between different molecules in a mixture are stronger than the interactions between the same types of molecules when they are pure. This leads to a decrease in vapor pressure compared to what is expected based on ideal behavior. In terms of the given options:

  • Benzene-acetone: Benzene and acetone do not exhibit strong intermolecular attractions that lead to negative deviation.
  • Benzene-chloroform: This pair shows negative deviation. Chloroform and benzene have strong hydrogen bonding and dipole-induced interactions when mixed, leading to a reduction in vapor pressure.
  • Benzene-ethanol: Ethanol tends to form hydrogen bonds with itself, and when mixed with benzene, it does not show strong enough interactions to cause negative deviation.
  • Benzene-carbon tetrachloride: No significant interactions cause negative deviation in this pairing.

Thus, the mixture benzene-chloroform demonstrates negative deviation from Raoult's law due to strong intermolecular forces such as hydrogen bonding between different molecule types.

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Approach Solution -2

Raoult's law states that the partial vapor pressure of each volatile component in a solution is directly proportional to its mole fraction in the solution. In real solutions, deviations from Raoult's law are observed. 
These deviations are classified into two types: 

- Negative deviation: This occurs when the intermolecular forces between the solute and solvent molecules are stronger than those between the solvent molecules themselves. As a result, the vapor pressure of the solution is lower than predicted by Raoult's law. 

- Positive deviation: This occurs when the intermolecular forces between the solute and solvent molecules are weaker than those between the solvent molecules themselves, leading to a higher vapor pressure than predicted by Raoult's law. 

In the case of benzene-chloroform, the intermolecular forces between the benzene and chloroform molecules are stronger than those between benzene molecules, leading to a negative deviation from Raoult's law. 

Thus, the correct answer is Benzene-chloroform.

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