Question

Calculate the vapor pressure (in torr) at 298 k in a solution prepared by dissolving 15.3 g of the non-volatile non-electrolye urea {co(nh2)2} in 107 g of water. the vapor pressure of water at 298 k is 23.76 torr. enter your answer to 2 decimal places

Answer 1

In colligative properties like vapor pressure, boiling point, freezing point and osmotic pressure, only the quantity of solute may affect their values. Their identity does not affect these properties. When solute is added to a solution, the vapor pressure decreases, that's why it is called vapor pressure lowering. The equation is

ΔP = xP°, where ΔP is the difference of the vapor pressure of the solution and of the solvent. P° is the vapor pressure of the pure solvent while x is the mole fraction of solute. The molar mass of urea is 60.06 g/mol while that of the water is 18 g/mol.

x =mol urea/total mol = (15.3/60.06)/[(15.3/60.06)+(107/18)]
x = 0.0411

Then,
ΔP = xP°=0.0411*23.76 = 0.976 torr

Therefore, the vapor pressure of the solution is 23.76 - 0.976 = 22.78 Torr.