A container is partitioned by a semi-permeable membrane into two compartments. O

Question

A container is partitioned by a semi-permeable membrane into two compartments. One compartment is filled with pure water, and the other is filled with saline solution (water + NaCl). The membrane only allows water molecules to move across. The container walls and membrane are rigid and so the volumes of the two compartments are fixed. At equilibrium, the molar fraction of NaCl in saline solution is 0.25, and the pressure of pure water is 1 atm. What is the pressure of the saline solution?

Assume that water and NaCl form ideal solution. The molar volume of pure water is 18 cm3/mol.

Explanation / Answer

mole fraction of solute = 0.25

pure water pressure = 1atm

for NaCl = i = 2

formula:

(Pure water pressure - solution pressure) / Pure water pressure = i x mole fraction of solute

(1 -solution pressure ) / 1 = 2 x 0.25

the pressure of the saline solution = 0.50 atm

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