Question

A solution is made by dissolving 24.1 g urea (), a nonelectrolyte, in 309 g water. (The vapor pressure of pure water is 23.8 torr at 25°C and 71.9 torr at 45°C.)

Calculate the vapor pressure of this solution at 25°C.

Vapor pressure =  torr

Calculate the vapor pressure of this solution at 45°C.

Vapor pressure =  torr

Answer 1

the main equation

P°new = P°solvent - dP

where

dP = xsolute * P°solvent

then

m = 24.1 g of urea

MW urea = 60

mol urea = m/MW = 24.1/60 = 0.4016

m = 309 g of water

MW wate r= 18

mol = m/MW = 309/18 = 17.1666

total mol = 17.1666 + 0.4016 = 17.5682

mol frac of solute = 0.4016 /17.5682 = 0.022859

then

at 25°

dP = xsolute * P°solvent = 0.022859*23.8 = 0.5440442 torr

at 45 °C

dP = xsolute * P°solvent = 0.022859*71.9= 1.6435621torr

then

P°(25°C) = P(25°C) - dP(25°) = 23.8 -0.5440442 = 23.25 torr

P°(45°C) = P(45°C) - dP45°) = 71.9-1.6435621 = 70.25 torr