Question

Negative liquid deviation from Raoult’s law is observed in which one of the following binary liquid mixtures?
(A) Ethanol and acetone
(B) Benzene and toluene
(C) Acetone and chloroform
(D) Chloroethane and bromoethane

Answer 1

Hint: The interactive forces of the solute and solvent molecules in the solution places a key role in deciding the deviation of the vapour pressure of the solution from the pure state. So, increasing and decreasing it accordingly.

Complete step by step solution:
Firstly, the Raoult's law for a mixture of liquid, states that the partial vapour pressure of each component in the mixture solution is proportional to the vapour pressure of the pure component, multiplied to its mole fraction.
Thus, the lowering of the vapour pressure of an ideal solution with non-volatile solute is directly related to the mole fraction of solute, with the interactive forces being uniform through the solute - solute, solvent - solvent and solute-solvent molecules. This may be the case when the size of the molecules is almost similar in both solute and solvent.
But this may not be possible in most of the cases, hence causing a deviation of vapour pressure in a non-ideal solution as the nature of interactions of molecules of solute and solvent changes. Thus, not obeying Raoult's law.
The negative deviation occurs when the interaction between solute – solute and/or solvent – solvent molecules becomes weaker than that of solute-solvent interaction.
So, in the case of a mixture of chloroform and acetone, it forms a hydrogen -bonding in solution, which increases the interactive force, making it difficult for them to escape. Thereby, contracting the volume of the solution and also lower the vapour pressure of the mixture solution.

Therefore, the negative liquid deviation from Raoult’s law is seen in the binary liquid mixture of the option (C)- chloroform and acetone.

Note: Whereas, in case of a mixture of Ethanol and acetone, the ethanol has hydrogen-bonding which breaks when acetone is mixed to it. Thus, it shows a positive deviation as the interactive force decreases and in a mixture of Benzene and toluene; and chloroethane and bromoethane, they are an ideal solution as both solute and solvent molecules have a similar structure.