Kaplan FL1, PS#19

[30somethingyearold]

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Question:
Solution X boils at 100.26°C and solution Y boils at 101.04°C. Both solutions are at atmospheric pressure and contain the same solute concentration. Which of the following conclusions can be drawn?
A. The freezing point of solution X is lower than that of solution Y.
B. The vapor pressure of solution X is higher than that of solution Y at 100.26°C.
C. Solution X and solution Y are immiscible.
D. The vapor pressure of solution X is lower than that of solution Y at 100.26°C

Answer is B.
The reasoning is because: "... A solution boils when its vapor pressure is equal to the atmospheric pressure, solution Y must have had a lower vapor pressure to begin with. Choice B is therefore correct, and choice D incorrect...."

I thought that since X boils at a lower temperature, it has a lower vapor pressure than Y. Why is it stating what it had to begin with and not now with the solute?! Thanks in advance!

 

[TheJourney]

Higher boiling point = lower vapor pressure. They are inversely proportional.
Since solution X had a lower boiling point its vapor pressure must have been higher.
I used to think vapor pressure and boiling point were directly related as well, after a few explanations to incorrect answers I realized they weren't the hard way as well.

 

[30somethingyearold]

Thank you! That makes sense.

To anyone else who is having the same problem, I also found my question posted previously on SDN and re-reading it on a different textbook also helped my understanding of this concept!