The following is adapted from a problematic question* asked on the Bio site. I would like to ask it free of those distractions here. If there is anything unduly artificial about the problem please advise and I'll try to revise the question. I think it belongs to physics at least as much as to chemistry.

Question: A bag whose walls form a semi-permeable membrane contains a mixture of 5% of solute A and 10% of solute B (both in water). The membrane allows passage of B particles but not A particles.

The bag is lowered into a solution containing a 10% solution of solute B. Instead of asking a multiple choice question let me keep it open-ended. What would we expect in terms of the movement of water/solutes in this situation? Can we predict, in rough terms, a final equilibrium state?

Thanks for any clarification!

*The problem is that one of the solutes was poorly chosen.

  • $\begingroup$ @DavePhD: Your answer seemed reasonable to me. Why the deletion? $\endgroup$
    – daniel
    Commented Jun 1, 2014 at 0:33
  • $\begingroup$ I wanted to think about the water for a moment. I added to the answer and undeleted. I would dialyze protein solutions in high concentration phosphate buffer against low concentration phosphate buffer back in grad school, and I remembered how the bags would pressurize at first because water would enter, so I wanted to add something about that. $\endgroup$
    – DavePhD
    Commented Jun 1, 2014 at 0:42
  • $\begingroup$ @DavePhD.. we were discussing about this problem of colloidal mixtures. How does a macromolecule affect osmosis? Or what macromolecules can give rise to oncotic pressure? $\endgroup$
    Commented Jun 3, 2014 at 5:47
  • $\begingroup$ @WYSIWYG: I think that merits a separate question. I wanted to keep this simple because (unless I am missing something) the answer here is correct and not consistent with the one at BioSE. I think if you expand your comment it's fine or perhaps ask a separate question (or I will). I expressly asked this one to avoid the colloid issue that was probably not intentionally in the original problem. Thanks. $\endgroup$
    – daniel
    Commented Jun 3, 2014 at 6:52
  • $\begingroup$ oh okay.. i am sorry. I can ask a separate question then. $\endgroup$
    Commented Jun 3, 2014 at 7:50

1 Answer 1


What would we expect in terms of the movement of water/solutes in this situation? Can we predict, in rough terms, a final equilibrium state?

Some amount of particles B will exit the bag, so as to equalized the osmotic pressure on both sides of the membrane.

The final state will be $<10$% B inside the bag and $>10$% B outside the bag.

The question does not specify whether the percentages are weight percent or something else. Without knowing more information such as molecular weight or molarity one can not make a quantitative determination of the final state.

Also, the question doesn't explicitly state that water can pass through the membrane, although such is usually the case. Water would initially enter the bag stretching the bag and raising the hydraulic pressure inside the bag. If the bag is elastic and not permanently deformed, extra water will eventually leave the bag, as osmotic pressure balances by B exiting the bag.


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