A solution is prepared by adding 35.00 g of lactose (milk sugar) to 110.0 g of water at The partial pressure of water above the solution is ________ torr.

Question

Vapor pressure at 55 degrees C is 118.0 Torr. The MW of lactose is 342.3 g/mol

Answer 1

You need to know the vapor pressure of pure water at the temperature of the experiment.  Neither vapor pressure nor temperature are given.  Assuming the temperature is 25ºC, you can look up the v.p. of pure water and it turns out to be 23.8 torr.

 

moles lactose = 35.00 g x 1 mole lactose/342 g = 0.102 moles

moles water = 110.0 g x 1 mole H2O/18 g = 6.11 moles

Total moles = 0.102 + 6.11 = 6.21 moles

mole fraction of H2O = 6.11/6.21 = 0.984

Vapor pressure of water (partial pressure of water) = 0.984 x 23.8 torr = 23.4 torr