Question

The vapor pressure of pure water at 25 °C is 23.8 torr. Determine the vapor pressure (torr) of water
at 25 °C above a solution prepared by dissolving 35 g of urea (a nonvolatile, non-electrolyte, MW =
60.0 g/mol) in 75 g of water.

where have i made a mistake?
35g/60=0.583
75g/18.02=4.162
0.583+4.162=4.745
4.162/4.745=0.88
but my practice test says the answer is 21 where do i go wrong?
please help...

Answer 1

You stopped too soon.
0.88 is the mole fraction of water. Multiply that by the normal vapor pressure of water (23.8) to obtain 20.87 which rounds to 21 mm.

Answer 2

oops, thanks for the help!