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When people talk about osmosis, they say that when we add a mole of NaCl, then each its molecule gets dissolved into 2 separate particles and therefore we have 2osM/L solution.

But both Na & Cl are attached to molecules of water which means that those water molecules are not water anymore and they can't penetrate the membrane. Thus not only the presence of Na & Cl itself changes the concentration of water in solution, but also the number of water molecules taken by those particles.

Therefore if we put 1 particle that binds to 1 water molecule and 1 particle that binds to 5 water molecules - they change the water concentration differently.

Please somebody tell me where I'm wrong.

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1 Answer 1

When you dissolve something in water, you build a solvation shell around this - so each ion (in the case of salts) gets surrounded by a number of water molecules. For more details read this Wikipedia article. The concentration change in free water molecules is usually not taken into account since it can be neglected due to the vast number of "unbound" molecules. So if you have one litre of water, it contains approximately 56 moles of water molecules. If you dissolve 1 mole of sodium chloride in this water, and take in account that this takes quite some water to get solvated, there is ways more water available. So the water concentration is almost constant (especially when you work at physiological osmosis condition).

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Okay, let's calculate this scenario: we add 1 mole of NaCl (~60g) into 56 moles of water. Each Cl atom binds to 6 atoms of water at average (according to wiki: en.wikipedia.org/wiki/Sodium_chloride). Let's suppose that Na takes up the same number of water molecules. Then 1 mole of salt takes 12 moles of water which doesn't look negligible to me. On practice the number will probably be lower because a single water molecule can be bound to both Na & Cl atoms which means that in highly concentrated solution 1 mole of NaCl will bind to 6 moles of water which is still not negligible. –  ctapobep Feb 5 at 7:56
    
It indeed sounds that its not neglectable. But you still have 80% of the water present. And: Water is a neutral molecule that can pass through the cell membrane, other dissolved substances cannot do this. –  Chris Feb 5 at 10:25
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