solutions & Raoult's Law

[TFS]

EK 30 minute test question.

When volatile solvents A and B are mixed in equal proportions, heat is given off to the surroundings. If pure A has a higher boiling point than pure B, which of the following could not be true?

A. The boiling point of the mixture is less than pure A
B. The boiling point of the mixture is less than pure B
C. The vapor pressure of the mixture is less than pure A
D. The vapor pressure of the mixture is less than pure B

The answer in the back of the book is B.

Can someone provide a good explanation for this? The explanation provided is so confusing! 😕

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[thefritz]

EK 30 minute test question.

When volatile solvents A and B are mixed in equal proportions, heat is given off to the surroundings. If pure A has a higher boiling point than pure B, which of the following could not be true?

A. The boiling point of the mixture is less than pure A
B. The boiling point of the mixture is less than pure B
C. The vapor pressure of the mixture is less than pure A
D. The vapor pressure of the mixture is less than pure B

The answer in the back of the book is B.

Can someone provide a good explanation for this? The explanation provided is so confusing! 😕

If the boiling point of A is 200 degrees and the boiling point of B is 100 degrees, how could the boiling point of the mixture be 50 degrees? It should be somewhere between 100 and 200 depending on the concentration of each in the mixture.

You should always think about conceptual problems with example numbers and extreme situations (very high and very low numbers).

 

[badmintondr]

Sorry, but could someone clarify why statement D would be correct as well? I thought vapor pressure exerted by a mixture of 2 volatile solutions was additive.

Thanks!

 

[unique135]

This one is tricky. Statement C/D is not correct (Both could be possible).

First, when both solvents are mixed, it gives off heat. So, this is exothermic (-delta H). The linear line of total vapor pressure in ideal conditions will be depressed forming a parabola-like curve in non-ideal conditions.


Draw a parabola (representing non-ideal total vapor pressure) and then erase one side a bit. Draw straight lines representing ideal partial vapor pressure for both solvent. Now, you can see why both solvents can have their ideal partial vapor pressure greater than non-ideal total vapor pressure