Question

Mixtures(s) showing positive deviation from Raoult’s law at ${35^{^0}}C$is(are):
A. Carbon tetrachloride + methanol
B. Carbon disulphide + acetone
C. Benzene + toluene
D. Phenol + aniline

Answer 1

Raoult’s law states that the partial pressure of each component of an ideal mixture of liquids is equal to the vapour pressure of the pure component multiplied by its mole fraction in the mixture.The causes for a positive deviation from Raoult’s law are their difference in polarity of liquids.

Complete step by step solution:
(a) Carbon tetrachloride( $CC{l_4}$) : It is a tetrahedral molecule since the carbon atoms form a single covalent bond with chlorine atoms by sharing their electrons. Since it is a tetrahedral molecule, they are symmetric and extend out in four different directions. Therefore, they are non-polar molecules.
Methanol( $C{H_3}OH$): It has both polar regions as well as the non-polar region. The polar region is the $- OH$ part attached to the carbon, the $O$ becomes partially negative and $H$ becomes partially positive. It is due to the uneven distribution of charge. Therefore, it forms a dipole-dipole interaction between them. Though it has both properties it behaves as a polar molecule.
Polar molecule and non-polar molecule typically tend to show positive deviation from Raoult’s law.

(b) Carbon disulphide( $C{S_2}$): It is a non-polar molecule because it has a linear structure, with the symmetric distribution of charge. There is a 180-degree bond between the carbon atoms, therefore any dipole moments arisen are cancelled out.
Acetone: It has both non-polar and polar but it is reasonably polar due to the $C = O$ part in the acetone, which can form a hydrogen bond with molecules.
Therefore, again the polar and non-polar molecule will show positive deviation from Raoult’s law.

(c) Benzene + toluene are examples of an ideal solution. They have a similar structure, therefore the force of attraction between them will also be similar. So, we call them as the ideal solution and Ideal solution doesn’t deviate from Raoult’s law as they follow Raoult's law.

(d) Phenol + aniline shows negative deviation. Since the A-B interaction is stronger than the A-A and B-B interaction. Therefore, they show slightly a negative deviation

**So option (a) and (b) are correct

Note:**
Some of the other points that we can keep in mind to identify the positive deviation are:
i. Association of Hydrogen bond in any one of the liquids
ii. Greater difference in molar mass of non-polar molecules
iii. Difference in polarity of liquids