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Homework Help: Average osmotic pressure of blood

  1. Jan 28, 2006 #1
    Aqueous solutions introduced into the bloodstream by injection must have the same osmotic pressure as blood; that is, they must be "isotonic" with blood. At 25C, the average osmotic pressure of blood is 7.7 atm. What is the molarity of an isotonic saline solution (NaCl in H2O)? Recall that NaCl is an electrolyte; assume complete conversion to Na and Cl ions.

    The equation I have for osmosis is π=MRT (π=pressure, M=molarity, R=constant, T=temperature), but in the text it says this is for nonelectrolytes. I'm not sure where to go with this.
  2. jcsd
  3. Jan 29, 2006 #2


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    Try looking into something called the van't Hoff factor.
  4. Feb 1, 2006 #3
    The equation you gave can be converted to the osmosis equation for associating/dissociating solutes by multiplying the RHS with a factor 'i' (Also Called the Van't Hoff Factor)

    So it becomes


    i is a factor defined as the number of particles after association/dissocation divided by the number of particles before it. One particle of NaCl Dissociates into two particles, The Sodium Ion and the Chloride Ion, so van't hoff factor for it is 2. replace i by 2 and proceed as usual.
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