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Is the Osmolarity definition in wikipedia wrong?

  1. Oct 10, 2009 #1
    Is the Osmolarity definition in wikipedia wrong???

    "Osmolarity is the measure of solute concentration, defined as the number of osmoles of solute per liter of solution (osmol/L). "

    but osmole mean particles that contribute to osmotic pressue. Meaning non permeable particles

    But osmolarity measures non permeable and permeable particles

    So is this definition wrong. Thank you :smile:
     
  2. jcsd
  3. Oct 10, 2009 #2

    Borek

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    Staff: Mentor

    Re: Is the Osmolarity definition in wikipedia wrong???

    ALL particles contribute to osmotic pressure, doesn't matter if they are permeable or not.

    --
     
  4. Oct 10, 2009 #3
    Re: Is the Osmolarity definition in wikipedia wrong???

    Osmotic pressure only defines non permeable particles. I sent you an pm. I'm confused!1

    this is the definition

    "Tonicity is a measure of the osmotic pressure of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in an external solution. Like osmotic pressure, tonicity is influenced only by solutes that cannot cross the membrane, as only these exert an osmotic pressure. Solutes able to freely cross the membrane do not affect tonicity because they will always be in equal concentrations on both sides of the membrane."
     
    Last edited: Oct 10, 2009
  5. Oct 10, 2009 #4

    Borek

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    Staff: Mentor

    Re: Is the Osmolarity definition in wikipedia wrong???

    Those that can cross the membrane will cross it till their concentrations (or more precisely their chemical potentials) on both sides of the memebrane will be identical. It doesn't mean their concentrations on both sides are always identical - that's only when solute is at equilibrium.

    Imagine two volumes of water divided with a mebrane that is permeable to both water and Na+/Cl-. You add some NaCl to one side. At first there is an osmotic pressure between both volumes, then it goes down as Na+/Cl- and water crosses the membrane. How long it will take depends on the membrane characteristics, but as long as equilbrium is not reached there will be measurable osmotic pressure.

    So, when calculating osmotic pressure, you have to take every particle into account, but at equilibrium those permeable can be ignored, as their effects cancel out.

    --
     
  6. Oct 10, 2009 #5
    Re: Is the Osmolarity definition in wikipedia wrong???

    Thank you :smile: I got it. However this is my last question

    "A solution can be both hyperosmotic and isotonic.[1] For example, the intracellular fluid and extracellular can be hyperosmotic, but isotonic - if the total concentration of solutes in one compartment is different than the other, but one of the ions can cross the membrane, drawing water with it and thus causing no net change in solution volume."

    So in osmotic solutions are they taking about the situation before equilibrium of permeable solutes and in tonicty they talk about the situation after equilibrium. Also how do solutes pull water with it. Is it because when one side has more solutes the water follow or are the solutes permanatly mixed with water.
     
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