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Chemistry Forums for Students => Inorganic Chemistry Forum => Topic started by: kimi85 on June 25, 2007, 10:26:25 AM

Title: colligative properties
Post by: kimi85 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.

Title: Re: colligative properties
Post by: Borek 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.

Title: Re: colligative properties
Post by: kimi85 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.:)