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The boiling point of an azeotropic mixture of water and ethyl alcohol is less than that of the theoretical value of water and alcohol mixture. Hence the mixture shows:
A. The solution is highly saturated.
B. Positive deviation from Raoult’s law.
C. Negative deviation from Raoult’s law.
D. Nothing can be said.

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Last updated date: 22nd Mar 2024
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MVSAT 2024
Answer
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Hint: Think about what the Raoult’s law is and what it can indicate. A positive or negative deviation from Raoult’s law is indicative of the interactions that occur among the 2 substances in an azeotropic mixture.

Complete answer:
To solve this question, we need to understand what an azeotropic mixture and Raoult’s law mean.
An azeotropic mixture is a mixture that has the same proportion for the vapour and fluid phases of both the substances present in the mixture.
Raoult’s law states that for ideal solvent, the partial pressure of the solvent in a solution or a mixture should be equal to the vapour pressure of the pure solvent multiplied by its mole fraction in the solution.
A positive deviation from Raoult's law is seen when the vapour pressure of the mixture is more than the expected or predicted vapour pressure, and a negative deviation is seen when the vapour pressure is less than expected.
Now we will see what a decreased boiling point implies.
We know that, whenever the boiling point of a mixture of two molecules is less, the interaction between those molecules are low. Here, the boiling point of ethyl alcohol and water is less means there is no good interaction between these molecules. If we consider ethyl alcohol as A and water as B. Here, in their pure form, that is A-A and B-B interaction is better than A-B interaction. Since the A-B interaction is weak, A-B bonds will be weak and they are far apart. So, the volume of mixture will increase. Since, A-B interaction is weak, the boiling point is low.
Since, the A-B interaction is less this means that the vapour pressure will be more than the expected value. They are more likely to switch to vapour state if the interaction is weak. Since the vapour pressure is more than expected, a positive deviation from Raoult’s law can be seen.
Therefore, the correct option will be ‘B. Positive deviation from Raoult’s law’

Note: Remember that a positive deviation in Raoult’s law means an increased vapour pressure and not the boiling point. A decreased boiling point will cause an increase in vapour pressure. Pay close attention to which quantity is decreasing and which is increasing.


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