Question

An aqueous solution of methanol in water has vapour pressure:
A.Equal to that of water
B.Equal to that of methanol
C.More than that of water
D.Less than that of water

Answer 1

Hint: To solve this question, you must recall Raoult's law. Raoult’s Law states that in a solution, the partial vapour pressure of each volatile compound is directly proportional to its mole fraction in the solution. The solution will show positive deviation from Raoult’s law.

Complete step by step answer:
Deviation from Raoult's law may either be positive deviation or a negative deviation.
Positive deviation is said to be shown in a mixture if the vapour pressure of the mixture is found to be higher than that expected on the basis of the Raoult’s law. It is observed when the cohesive forces between the similar molecules of each liquid are greater than the adhesive forces between the molecules of different liquids.
That is A-A and B-B interaction > A-B interaction

Negative deviation is said to be shown in a mixture if the vapour pressure of the mixture is found to be lesser than that expected on the basis of the Raoult’s law. It is observed when the cohesive forces between the similar molecules of each liquid are weaker than the adhesive forces between the molecules of different liquids.
That is A-A and B-B interaction > A-B interaction.

Methanol and water form hydrogen bonds between them. This hydrogen bonding is stronger than that between methanol molecules but the interactions are not as extensive as in water.
Thus, it has a vapour pressure lower than methanol and higher than water.
Overall A-A and B-B interaction > A-B interactions and the solution shows positive deviation.

Hence, the correct option is C.

Note:
Raoult’s law is given for an ideal mixture. A solution that shows a deviation from Raoult's law over the entire range of various compositions of the mixture, the solution is said to be a non- ideal or real solution. In a real solution, the pair of liquids that constitute the mixture do not have a uniformity throughout attractive forces, which may be either adhesive or cohesive. Thus, they show a deviation from Raoult's Law.