At 35 degrees Celsius, the vapor pressure of a compound 'A' is 512mm Hg and of acetone is 344mm Hg. A solution of compound 'A' and acetone in which the mole fraction of compound 'A' is 0.25 has a total vapor pressure of 600mm Hg. Which of the following statements is true for this system and why?

A) A mixture of 100ml of acetone and 100ml of 'A' has a volume of 200ml.

B) When acetone and 'A' are mixed at 35 degrees Celsius, heat is absorbed.

C) When acetone and 'A' are mixed at 35 degrees Celsius, heat is released.

D) A mixture of 100ml of acetone and 100ml of 'A' will have a volume less than 200ml.

I'm don't understand this question...

Raoult's law says that the vapor pressure of the entire system is equal to the sum of the vapor pressures of the pure components times its mole fraction.



where is the total pressure in the system,
is the vapor pressure of pure substance #1 and is its mole fraction in the mixture,
and
is the vapor pressure of pure substance #1 and is its mole fraction in the mixture, and so on.

Of course, this is only valid at the temperature at which are measured, and it is only strictly valid for "ideal gases."

Judging from the question, first calculate the expected total vapor pressure using Raoult's law. I'll give you a hint. It's not even close to 600 mm. This should be obvious since the vapor pressures of both pure substances are less than 600 mm. So, you have to try and figure out what is happening. Once you do that, evaluate the four answers and determine which one is correct. (A couple of the possibilities look like they're just trying to throw you off the track).

Quote:

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Originally Posted by Perito

Raoult's law says that the vapor pressure of the entire system is equal to the sum of the vapor pressures of the pure components times its mole fraction.



where is the total pressure in the system,
is the vapor pressure of pure substance #1 and is its mole fraction in the mixture,
and
is the vapor pressure of pure substance #1 and is its mole fraction in the mixture, and so on.

Of course, this is only valid at the temperature at which are measured, and it is only strictly valid for "ideal gases."

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So, if we consider temperature to be constant, then that strikes out options B & C..
And as this is a nonideal solution, the sum of initial volumes can't be equal to final volume of mixture.
So is the correct option D?

I'm not sure why you'd assume the temperature to be constant.

Option D) is probably true in any case, but I'm not sure you have enough information to verify that here. Heck, I'm not sure you have enough information to say any of the four is true, but I wouldn't have picked D)

Quote:

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Originally Posted by Perito

I'm not sure why you'd assume the temperature to be constant.

Option D) is probably true in any case, but I'm not sure you have enough information to verify that here. Heck, I'm not sure you have enough information to say any of the four is true, but I wouldn't have picked D)

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Then, maybe, the correct option is A because the rest of them indicate a nonideal solution, while A indicates an ideal solution. That's the best I can guess...

I would assume that a spontaneous chemical reaction had taken place and therefore, heat is released. In this case, I would assume that the total volume would be less than 200 mL (but probably not by very much).

I will state, however, that the problem can be ambiguous. There may be factors that I've forgotten in my years as a chemist, or that I never learned ;-)

Quote:

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Originally Posted by Perito

I would assume that a spontaneous chemical reaction had taken place and therefore, heat is released. In this case, I would assume that the total volume would be less than 200 mL (but probably not by very much).

I will state, however, that the problem can be ambiguous. There may be factors that I've forgotten in my years as a chemist, or that I never learned ;-)

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No prob, actually I was solving some past exam papers and found this weird question among others.