Smyle4Jenn

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Can someone explain this problem:

Beaker A contains a volative solvent and Beaker B contians a 10% solution of a nonvolatile solute in the same volatile solvent. Both beakers are placed in a closed environment. Which is true once the system reaches equilibrium?

A) Beaker A will contain all the solvent
B) Beaker B will contain all the solvent
C) Beaker B will contain 90% of all the solvent
D) Both beakers will contain the same amount of solvent

The answer is B. I understand that Beaker B has a lower vapor pressure, but why would all the solvent go to B? I thought that B would contain more solvent than A.
 

KitEKatEli

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I think this problem is more about osmotic pressure than about Raoult's law. The classic example of osmosis is a divided container (think U-tube) containing pure liquid on one side (A) and dissolved solute on the other side (B). They are separated by a barrier that is permeable to the liquid but not to the solute. The setup in the problem is pretty much the same thing. And what happens in the U-tube? Pure liquid travels across the barrier until the osmotic pressure on side B opposes the tendency for liquid to go from A to B. Since both beakers are open in the problem, there is no comparable pressure, so all the liquid in A is allowed to travel to B. That's my hand-waving explanation, anyways.

Honestly, I didn't know the answer when I read it. But you could have used process of elimination to solve this problem. You know that B will have more solvent than A, so you can eliminate choices (A) and (D). When I read answer choice (C), it seemed like a "pick me" answer because of the 10%-90% relationship. But if you think about it for a sec, there's no intuitive reason why B should have 90% of the solvent. I mean, there's 90% solvent and 10% solute in B, but you also have to consider A, which is 100% solvent. If everything is in grams, then (C) is saying that B would have 90% of 190 g, and why should that necessarily be true?