[ap90r]

Hello,
     I have a question about how to convert osmolality to osmotic pressure. I tracked down an osmometer and used it today, but turns out it measured in osmolality instead of osmotic pressure. I'm wondering how I could convert between the two. I've been searching around but can't find anything.

I was thinking I could isolate the van't Hoff constant by comparing the osmolality to the molarity of the solutions, but turns out there's an osmotic coefficient, which blew that option out of the water.

Any advice you could give me would be greatly appreciated. I'm not sure where to go next with this. Any useful equations or advice would be awesome.


Thanks,
Adam


Note: I forgot to mention, the results I got back from the osmometer we're a bit interesting.

For NaCl
Molarity:          Osmolality (what osmometer measured):
70.2 mmol       46 milliosmoles/kg
147 mmol        242 milliosmoles/kg
315 mmol        497 milliosmoles/kg

For the first measurement, there seem to be fewer solute particles than the number of moles of NaCl in the solution (I realize one is molarity and the other is molality, but it's water so they should be fairly close). That doesn't make sense, as you'd expect there to be at least more osmoles than the original molarity solution, considering NaCl would dissolve into more solute particles. Then it goes up with increasing molarities. Same thing happened the the several sodium sulfate solution concentrations I measured.