Question

The graphical representation of vapour pressure of two component systems as a function of composition is given alongside. By graphic inspection, answer the following question:
Name the type of deviation shown by this system from Raoult’s Law.

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.

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.
In the graph we can see clearly that the observed vapour pressure is lesser than the expected vapour pressure.

Thus, the system shows negative deviation from Raoult’s law.

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.