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)?