Ideally, all solute particles in solution affect solvent activity,
and osmotic solutions are measured in osmolal units (osm). A 1
osm solution of a non-dissociating solute (such as glucose) is
numerically equivalent to 1 m, while a 1 osm solution of an electrolyte
consisting on a cation and an anion (such as NaCl) is 0.5 m. Why?
Because all the component ions of a salt form shells of water,
and thus a di-ionic solute such as NaCl forms twice as many osmotically
active particles in solution than one that doesn't dissociate
(such as glucose). Thus, osmolality, along with vapor pressure,
freezing point depression and boiling point elevation, is a colligative
property of a solution.
In practice, at the low solute concentrations characteristic of
most cells and their environments, osmotic calculations are simplified
from osmolal to osmolar units (and from molal to molar units).
What would be the ideal osmolarity of 5 mM MgCl2? of a 10 mM
phosphate buffer at pH 7.0? of a physiological salt solution
containing 130 mM NaCl, 10 mM glucose, 6 mM KCl, 2 mM MgCl2,
1 mM CaCl2, and 10 mM phosphate buffer (pH 7.0)?