Question

The following plot shows the vapor pressure of various solutions of components A and B at some temperatures.

Which of the following statements is false concerning solutions of A and B?

a. The solutions exhibit negative deviations from Raoult’s law.

b. ${\mathbf{\Delta H}}_{\mathrm{soln}}$for the solutions should be exothermic.

c. The intermolecular forces are stronger in solution than in either pure A or pure B.

d. Pure liquid B is more volatile than pure liquid A.

e. The solution with ${\mathbf{\chi }}_B\mathbf{=}\mathbf{0}\mathbf{.}\mathbf{6}$ will have a lower boiling point than either pure A or pure B.

Short answer

Only one statement is false that is the solution that has a mole fraction of B, 0.6 will have a low boiling point as compared to the pure solution because the vapor pressure of the mixed solution will be less as compared to its pure A or B because the solution shows the negative deviation and boiling point, and vapor pressure are inversely related to each other. Therefore, the boiling point of the solution will be higher than pure A or pure B.

Step-by-step solution

Subpart (a): Deviation according to Raoult’s lawStep 1:

When the mole fraction of B starts increasing from zero to one, the vapor pressure of pure solution A starts decreasing, whereas the vapor pressure of pure solution B starts increasing, this shows that the vapor pressure of pure solution will be greater as compared to that of the mixture of two solvents or solutes, this shows that solution will exhibit negative deviation.

Subpart (b): Change in enthalpyStep 2:

As the bond between A and B is stronger than the pure A-A and pure B-B. So, the solution with negative deviation, when a new bond is formed there is the release of heat and the process is called the exothermic process. Therefore, the value of will be less than zero and negative.

Subpart (c): Interactive forcesStep 3:

According to Raoult’s law, in case of negative deviation, the intermolecular bond that is between A and B will be stronger as compared to the bond between A-A molecule and B-B molecule due to which the vapor pressure of the solution also decreases.

Subpart (d): Volatile substanceStep 1:

The solutes that are volatile can easily escape from the solution to become vapor, that’s why the volatile solutes have higher vapor pressure as compared to the non-volatile solute.

In the given graph, it is clear that the vapor pressure of pure liquid B higher than that of pure liquid A which means B will be more volatile than A and can easily convert into vapor form.

Subpart (e): Step 1:

According to Raoult’s law, when a solution shows negative deviation that means the vapor pressure of the mixed solution will be less as compared to its pure solvent or solute. Thus, if the vapor pressure is low then the boiling point will be high because they are inversely related to each other.

Therefore, the solution with a mole fraction of B, 0.6 should have a higher boiling point than pure A or pure B because it has lower vapor pressure than pure A or pure B.