Question 42

[kazuma]

42. The vapor pressure of water at 25 oC is 23.6 mmHg. If 0.180 kg of glucose is added to 0.900 kg of water, what is the vapor pressure of this solution?

1. 23.1 mmHg [x]
2. 24.1 mmHg [x]
3. 22.5 mmHg [x]
4. 22.2 mmHg

are the answers gonna be the close to each other on the mcat?

[admin]

It is definitely not common but there have been questions where the answer choices have had a difference of 0.002. Our aim is to get you ready for anything they can throw at you.

[srinivasan5618]

I understand how to arrive at the mole fraction, of solute but I don't understand what to do with it. Why are we subtracting it from 23.6/51??

[jellywing_2058]

This is a colligative property of an aqueous solution (vapor pressure).

This relationship is also known as “Raoult’s Law”, which states that the partial pressure of a solvent (H2O) over a solution (H2O + glucose) or P1, is given by the vapor pressure of the pure solvent (P°) times the mole fraction of the solvent in the solution (X1). So, P1 = (X1)(P° x 1).

In a solution containing only one solute, X1 = 1 – X2, where X2 is the mole fraction of the solute.

Therefore, replacing X1 with 1-X2, gives a final equation of change in pressure (P) = (X2)(P1).

[Guest]

This question seems ridiculously technical. I would have assumed you would have worked the question out like so ---

P(of lowered water)= (Mole fraction of water)P(of pure water)

So you have:

(50/51)(23.6) = answer

I agree with anyone else in this blog, that these answers are way too close. Preparing for very rare questions is not efficient in my opinion. Realistically, I would imagine over 75% of us taking the MCAT will never see answers that are this close and as many calculation questions.

This post will probably get deleted but I think its worth adding that I frown upon GS tests overall. For several reasons,
1. Too many calculations.
a. there should be more qualitative questions, but that would take you guys too much time and too much money.
2. Too many repeat questions.
a. more than a handful of times on BS and PS sections ive seen repeat questions that are re-donk-ulous. I'm definitely not getting my moneys worth.

I have more complaints, but I'm starting to realize this rant is going to be in vain.

[mcat_premed3832]

repeat questions?

where?

out of 1500 questions, there can't be more than 20 repeats! (less than 2% if that much)

[thaertlein2125]

Is the exclusion of glucose's molecular formula meant to add to the obscurity of this problem, making it really rare, or was it just overlooked as a common known piece of information?

I am surprised that for a normal level question we are expected to know the molecular formula of glucose. While this is not a tough fact to retain, it is not pertinent to the question theme (vapor pressure and mole fraction).

With regards to the closeness of the answers, if you get the equation right there is no way you should end up with a wrong answer. I have seen these types of answers on AAMC and Kaplan practice MCATs too.

[hua8986059]