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Osmotic pressure

  1. Nov 27, 2013 #1
    osmotic pressure is the pressure difference between two solutions of different solute concentrations separated by a semi-permeable membrane.

    my book says: "the osmotic presssure is exactly the same as the pressure of an ideal gas of the same concentration as the solute. in fact, it's tempting to think of the osmotic pressure as being exerted entirely by the solute, once we have balanced the pressure of the solvent on both sides. This interpretation is bad physics."

    why is it incorrect to say that the osmotic pressure comes from the solute?? fundamentally, at the site of the SP membrane, the solvent can pass through and the solute cannot, and the osmotic pressure is essentially the difference in the partial pressures exerted by the solvent molecules on each side. This difference comes from the fact that not all of the molecules hitting the SP membrane are the solvent molecules, some of them are the solute molecules which cannot pass through. So I don't see how it's bad physics to say that the solute is what's exerting the osmotic pressure (the pressure difference)

  2. jcsd
  3. Nov 28, 2013 #2


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    Staff: Mentor

    This cries for a next phrase with an explanation, is it really the whole related quote?
  4. Nov 28, 2013 #3
    i think it would be easier to illustrate the context with a picture.


    (the paragraph that starts with "figure 5.78")

    based on your response, i'm guessing that i do have the right idea of osmotic pressure and i am not misguided about where it comes from?
  5. Nov 28, 2013 #4


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    Staff: Mentor

    There won't be the osmotic pressure without a solvent, so it is not solute only that matters.
    Last edited: Nov 28, 2013
  6. Nov 28, 2013 #5


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    Osmotic pressure is only pressure resulting from partial pressures. People tend to treat osmosis as something a bit 'magic but there's no extra force from anywhere. The 'small molecules' of the solvent will spread out into the whole of a container but the larger molecules will stay 'inside' the membrane. Once equilibrium is reached, you will have the pressure of just the solvent on one side but the pressures of the solvent plus the solute on the other side of the membrane. Hence the membrane experiences a net pressure towards the dilute side. There will be solvent on both sides - it will find its way into the solute, eventually, by diffusion.
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