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I may not understand osmosis very well. Let us suppose two compartments filled with water, separated by a semi-permeable membrane. At equilibrium, both levels are equals. Let us introduce now a given volume of solute in one of the compartments (say right). At first, the level of the right compartment will increase, to accommodate the extra volume of solute. Because of this, the concentration of water in the right compartment has decreased, and is no more at equilibrium with the left compartment. Thus a net flux of water from left to right occurs until equilibrium is reached.

That's what I understand from what I read so far, but I have a problem with this. In this particular example, it is true to say that the water concentration on the right side decreased when the solute is introduced, but because of the excluded volume (from the solute), the water pressure should remain the same, and thus I would not expect a net flux.

Unless there is other effect I don't consider? Thank you for your explanation.

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The partial pressure of the water in the solution does, indeed, decrease. The total pressure of water plus solute, i.e. the pressure of the solution as a whole, stays the same.

Is this what you are asking? How can we see this? Pressure is force per area. Since we didn't change the area, and because forces from different atoms/molecules in the solution are additive, the partial pressures are additive. This is called Dalton's Law. Strictly speaking, this is only true, if there are no internal forces between the components of a mixture, so it holds reasonably well for many gas mixtures at low pressure and high enough temperature, but it does not hold for concentrated ionic solutions, for which we also have to calculate strong interactions between the solvent atoms/molecules and the solute ions.

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  • $\begingroup$ Yes, it was a misconception I had, thanks for clarifying. $\endgroup$ – David Sep 1 '14 at 9:01

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