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Biology - Diffusion/Osmosis of 2 Solutions

  1. Oct 2, 2009 #1
    1. The problem statement, all variables and given/known data
    You have two solutions - Solution A = 0.5 M NaCl and 0.5 M KCl Solution B= 1.0 M NaCl

    If Solution A and B are seperated by
    a) a membrane permeable to all three ions what will happen?
    b) a membrane that ions can not pass through but that water can, what will happen?

    2. Relevant equations


    3. The attempt at a solution

    I'm pretty sure Part A is talking about passive transport/diffusion of two solutes. I know that 2 different solutions will end up traveling through the membrane until they reach equilibrium.

    I'm also certain that Part B is talking about passive transport/osmosis, in which case the water would diffuse from the solution with less concentrated solution to the solution with more concentrated solution.

    What I don't understand is how to use the moles of the given solutions to help me figure out what the right answer is. We had a question like this on a quiz that I got wrong and then also it is now on my homework, and he has yet to mention anything like this in class, so I don't know how to apply the knowledge of diffusion/osmosis to this question.
  2. jcsd
  3. Oct 2, 2009 #2


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    Part A. On one side of the membrane you have a mix of .5M NaCl and .5M KCl. That solution is actually .5M Na+, .5M K+ and 1.0M Cl-.

    On the other side of the membrane you have 1.0M NaCl which is actually 1.0M Na+ and 1.0M Cl-. So, one side is relatively rich in Na+ one side is relatively rich in K+ and both sides are equi-osmotic with respect to Cl-. The membrane can freely pass Na+, K+, Cl- and water.

    Can you take it from there?

    Part B. Yep.
  4. Oct 2, 2009 #3

    Ok. I was able to figure out Part A while I was waiting for a reply. It was actually a multiple choice question with instructions to choose the correct answer or answers.

    So the choices for Part A were: (my answers are bolded)
    A. K+ will diffuse from A to B
    B. Na+ will diffuse from A to B
    C. Na+ will diffuse from B to A
    D. Cl- will diffuse from B to A
    E. Cl- will diffuse from A to B
    F. Cl- will not diffuse.

    For Part B the choices are:
    A. Water will move from A to B
    B. Water will move from B to A
    C. Water will not move in either direction.

    So- Both solutions are 2.0M total concentration. But Solution A has 2 different pieces in its solution whereas Solution A is only NaCl....

    How do I figure out which side is more concentrated? Would water be moving to both sides to account for the lack of K in Solution B and the lack of Na in Solution A?
  5. Oct 2, 2009 #4
    Ok- or would water not be moving at all? Since a mol isn't really a number it would be the same thing as saying "a dozen" of something. So- if I have apples(Na), oranges(Cl) and pears(K) I could say...

    Solution A- has 1/2 dozen apples, 1/2 dozen pears and a dozen of Cl.

    Solution B - has 1 dozen apples and 1 dozen oranges

    Both of those solutions equal out to 2 dozen... or 2.0M.... so they are concentrated the same?

    I don't know... this just confuses me because in class we only talked about ONE type of solute of different concentration- not more than one. =/
  6. Oct 2, 2009 #5
    Here's a tip for these sorts of problems: Draw it out. It helped me get the idea when I was learning it.

    Just draw both sides in the initial phase with their concentrations. Then take a look at what can move through the membrane. Once that is established, just see which side has a lower concentration of that particular substance and that's the direction it will move.

    Not sure if that helps for this situation though. Like you my lessons only entailed two substances, a miscellaneous solute and water.
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