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Question

An aqueous solution of sucrose has an osmotic pressure of 250000 Pa at 298K.

Calculate the molar concentration of the sucrose

Calculate the vapour pressure of the solution at 298 K if the vapour pressure of pure water at this temperature is 3170 Pa and assuming ideal solution behaviour

Answer 1

p = 2.47 atm

2.47 = molar concentration x 0.0821 x 298

molar concentration = 0.101 M

this means 0.101 moles sucrose in 1 L of solution.

Mass sucrose = 0.101 mol x 342.3 g/mol = 34.6 g

assuming density = 1 g/mL

mass solution = 1000 g

mass water = 1000 g - 34.6 = 956.4

moles water = 956.4 g / 18.02 g/mol =53.6

mole fraction water = 53.6 / 53.6 + 0.101 = 0.998

p = p°X

p = 3170 x 0.998 = 3164 Pa