When people talk about osmosis, they say that when we add a mole of NaCl, then e

Question

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.

Explanation / Answer

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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