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Topic: colligative properties  (Read 3665 times)

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Offline kimi85

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colligative properties
« on: June 25, 2007, 10:26:25 AM »
I don't know how to calculate the number of moles from the mole fraction.

This is the problem: What is the concentration of solute in an aqueous solution whose vapor pressure , at 20 degree Celsius, is 17.0 mm Hg?  the vapor pressure of water, at 20 degree Celsius, is 17.4 mm Hg.

I used this formula: deltaP = xPsolvent; x is the mole fraction of solute.  I calculated the x and from there I don't know how to solve the number of moles of the solute, since I don't know the number of moles of water.

Thank you for anyone who can help.

Offline Borek

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Re: colligative properties
« Reply #1 on: June 25, 2007, 10:35:03 AM »
Molar fraction is already a way of expressing concentration.

Does the question specify answer units? Molar? Molal? Percentage? If so, you will need more information, like solution density and/or molar mass of the substance.
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Offline kimi85

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Re: colligative properties
« Reply #2 on: June 25, 2007, 10:47:59 AM »
Thanks so much. I think you're right.

But the correct answer is 1.308 moles. I think there's something missing in the problem. 

Thanks again.:)

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