# A simple osmosis problem

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.

• @DavePhD: Your answer seemed reasonable to me. Why the deletion? – daniel Jun 1 '14 at 0:33
• 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. – DavePhD Jun 1 '14 at 0:42
• @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? – WYSIWYG Jun 3 '14 at 5:47
• @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. – daniel Jun 3 '14 at 6:52
• oh okay.. i am sorry. I can ask a separate question then. – WYSIWYG Jun 3 '14 at 7:50

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