Finding mole fraction given vapor pressure?

I'm studying for my AP Chemistry semester exam and came across a question that I don't know how to answer. The question is:

A solution of urea (CH4N2O, molar mass= 60.05 g/mol) in 120 mL of water has a vapor pressure of 19.6 torr at 25 degrees Celsius. What is the mole fraction of the urea in the solution? (The vapor pressure of pure water is 23.8 torr at 25 degrees Celsius.)

Please show me how to answer the question, not just the answer!

And thank you for your time :)

Hyperion2014-01-13T19:35:54Z

Favorite Answer

PV = nRT
43.4(0.12L) = x (.082)(298 Kelvin)
x = .21 moles

19.6/43.8 = 0.45 partial pressure

.45 * .21 = 0.09 mole

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