Question

Negative 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: We must remember that the equilibrium between a liquid and its vapor produces a characteristic vapor pressure for each substance that depends on the temperature. The lowering of the vapor pressure is caused by a lesser ability of the solvent to evaporate, so equilibrium is reached with a smaller concentration of the solvent in the gas phase. The vapor pressure of a solution is expressed using Raoult’s law:
${P_{solv}} = {\chi _{solv}}{P^o}_{solv}$
The vapor pressure of the solvent $\left( {{P_{solv}}} \right)$ above a dilute solution is equal to the mole fraction of the solvent $({\chi _{solv}})$ times the vapor pressure of the pure solvent$\left( {{P^ \circ }_{solv}} \right)$.

Complete step by step answer:
Positive deviation from Raoult’s law is seen when the observed vapor pressure is greater than expected vapor pressure and it occurs when the A-B attractions are weaker than the average of the intermolecular attractions in the pure constituents of the mixture.
Negative deviation from Raoult’s law occurs when the intermolecular forces between the dissimilar molecules are stronger than the average of the intermolecular forces in the pure substances.
Mixtures of ethanol and acetone show positive deviation because the interactions are weaker in acetone and ethanol when compared to interactions between ethanol and ethanol.
Therefore, option (A) is incorrect.
Mixtures of benzene and toluene do not show any kind of deviation, as the types of intermolecular attractions are same to those in the liquid components. Hence, the mixture is an ideal solution.
Therefore, option (B) is incorrect.
Mixtures of chloroform and acetone show negative deviation because the intermolecular attractions between chloroform and acetone are greater than the intermolecular attractions between the molecules of chloroform (or) between the molecules of acetone.
Therefore, option (C) is correct.
Mixtures of chloroethane and bromoethane do not show any kind of deviation, as the types of intermolecular attractions are same to those in the liquid components. Hence, the mixture is an ideal solution.
Therefore, option (D) is incorrect.

Therefore option C is correct.


Note:
We have to know an ideal solution obeys Raoult's law at all temperatures and pressures. The solute-solute and the solvent-solvent interactions are almost the same as solute-solvent interactions. No association or dissociation occurs. It does not form an azeotrope mixture. When the intermolecular forces of attractions are similar or equal, we get an ideal solution.