Question 5

[priya.bork7031]

5) What would the freezing point in Kelvin of a solution which is 0.50 molal in sucrose and 0.50 molal in acetic acid be?
(Kf of water = 2.0 oC mol-1 and freezing point of water = 0 oC)

1. -1.0 K[x]
2. -2.0 K[x]
3. 272 K[x]
4. 271 K

Although I understand the explanation I wanted to just clear something up for myself. The 0.5 molal acetic acid, if it were a strong acid and dissociated to 0.5 molal H+ and 0.5 molal conjugate base, only the conjugate base affects the freezing point depression?

Or are we assuming here that the acetic acid is the only thing that is affecting freezing point depression? I was a bit confused that perhaps I needed to use 1.5 molal instead of only 1.0 molal


Thank you so much for the reviews and tests by the way. I feel like they have really targeted my strengths and weaknesses (they want me to put off the MCAT, but that's another story )

[nedaa.asba6809]

No, not just the conjugate base because colligative properties depend on the number of particles not the type of the particles, so a single proton like H(+) is equivalent in importance to a bigger molecule like a acetate group.

[sfarukhi1817]

i dont quite understand why you add 0.5 m acetic acid +0.5m sucrose?

[stephaniem3562]

Acetic acid and sucrose are non volatile solutes that are added to water. Hence you add up their molalities. What I dnt understand is why not take the "i" into consideration? i for acetic acid is 2 and for glucose, i=1. Therefore total i= 3. Thus change in temp= Kf* (0.5m+0.5m)*3.

Why is this not correct?

[mcat_premed3832]

This problem aims for a concept often tested on the MCAT (though it's name is rarely given on the test!): Colligative properties.

These are properties that depend on the number NOT the type of particles.

[ebsincla3909]

For this question, the freezing point depression was 2 degrees C right? The inital freezing point of 0 is converted to Kelvin making it 273K, but wouldnt the freezing point depression of 2 degrees C also have to be converted to Kelvin? Making it 275, and making the answer -2? Maybe im just not understanding it correctly, and the value of 2 is unitless...not sure--Thanks!

[tnfifn3735]

[jellywing_2058]

No the answer given is correct.
Sucrose as per the passage (not glucose) is not an electrolyte and is a sugar (organic molecule) that does not ionize and so the Van Hoff factor (i) for sucrose (or even glucose) will be = 1 (not 2).

So, for sucrose i = 1 and the answer is in fact D as given and the explanation is fine.

[tnfifn3735]