Because the solvent, water, also contains the hydroxide ion, OH-, it may be considered that the effect of NaOH ionization is partially canceled in the osmolarity.

What I want to do is to replicate the andere, so I think I need the Kb and cakculate thé fraction of OH-. Look for NaCl is simple, but for NaOH m'y attente was not succesful.

That's not so difficult: the Buffer preparstion department makes 0.5 M NaOH and sends thé sample to a certified lab to déterminé thé Osmolarity. A 0.1 M NaOH solution f.i. gives 0.2, so completely splits nto ions.