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
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
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