[MaxiKing]

Do H₃O ions increase osmotic pressure in water solutions of acids? My medical chemistru teachers states that’s not but I dont understand it. Could you explain it to me? My professor says thats 1 mole of HCL in water has only 1 Osm. Is it true?

[Borek]

Well, I already commented on the question at PF, so it is hardly a second opinion - but IMHO it is completely wrong. No source I am aware of ever stated that, all just treat acids as every other dissociating substance producing ions, and all produced ions are equivalent.

Plus, whoever states that H⁺ react with water molecules producing H₃O⁺ and that makes the H⁺ unique is just ignoring whole chemistry of interactions between dipoles of water molecules and ions in the solution (especially cations). Every ion is solvated - the smaller the ion, the higher its charge, the more water molecules are involved and the stronger the bonding. For H⁺ this effect is the most pronounced, but even there it is actually not H₃O⁺ that is produced, more like whole series of H(H₂O)_n⁺ cations, in a series of consecutive equilibrium reactions.

Broadly speaking it is this ordering effect ions have on surrounding water molecules that is responsible for all colligative properties (more ordered water molecules around ions means less "free" water molecules doing water molecules business), and H⁺ is in not different from every other ion present.