bellow vapour pressure question

[destroythemcat]

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I am confused by this answer can anyone help explain?

Q: The VP of a liquid compound at 25C is 100 torr. at the same temp. and under a pressure of 80 torr the compound will be:

A: a. solid b. liquid c.a gas and liquid in equilibrium d. a liquid and solid in equilibrium

-->answer is C

I don't understand why the answer is not liquid???

 

[Czarcasm]

I am confused by this answer can anyone help explain?

Q: The VP of a liquid compound at 25C is 100 torr. at the same temp. and under a pressure of 80 torr the compound will be:

A: a. solid b. liquid c.a gas and liquid in equilibrium d. a liquid and solid in equilibrium

-->answer is C

I don't understand why the answer is not liquid???

Well, the first thing you want to realize is that only temperature and the intermolecular forces of the solution are the determinants of vapor pressure. Because at some temperature (25C here), the vapor pressure presiding above the solution is 100 torr, ... if you maintain the same temperature, regardless of whatever the new external pressure will be, the vapor pressure will still be 100 torr. However, you may recall that when vapor pressure = atmospheric pressure, the solution will boil. Because we are in excess of 80 torr (the atmospheric pressure), this must mean we have already surpassed the liquid-gas equilibrium, which essentially means the solution exists entirely as a gas at this point. Hence, the answer C (gas) seems the most appropriate.

 

[el_duderino]

Answer (C) is "a gas and liquid in equilibrium"

I'm a little confused as to why it wouldn't simply be "gas." If the vapor pressure is higher than the ambient pressure, all of the liquid should vaporize (it'll boil away).

 

[sillyjoe]

Answer (C) is "a gas and liquid in equilibrium"

I'm a little confused as to why it wouldn't simply be "gas." If the vapor pressure is higher than the ambient pressure, all of the liquid should vaporize (it'll boil away).

For this question C is the better answer. Remember 1 ATM is 760 Torr. The difference between 80 torr and 100 torr is small enough that there will be an equilibrium between the gas and the liquid.

 

[destroythemcat]

Czarcasim: if the VP is 100 torr and the Atm pressure is 80 torr --> we are past the boiling point and everything is gas. why is the answer not gas then?

 

[Czarcasm]

Czarcasim: if the VP is 100 torr and the Atm pressure is 80 torr --> we are past the boiling point and everything is gas. why is the answer not gas then?

Unfortunately, that's just a matter of reference. It's difficult to really say since I don't have much information. However, I'm assuming here that they're considering a solution which is still in the process of evaporating (ie. it's boiling). In other words, by decreasing the atmospheric pressure, it will begin to evaporate gradually but hasn't quite reached the point where the liquid is fully evaporated. Regardless of the true reasoning, choice C is the most appropriate answer. Often times, you may encounter a question with an answer you don't necessarily agree with or understand fully, but typically it's the only choice appropriate to answering the question.

 

[el_duderino]

C is simply wrong. A boiling liquid is not in equilibrium.

I'm guessing this isn't from an AAMC test. It's a bad question.