mosmolL

MosmolL, or milliosmoles per liter, is the unit used to express osmolar concentration in a solution. It measures the total number of solute particles per liter, adjusted for dissociation into ions. The symbol most often encountered is mOsm/L, although mosmolL is a variant spelling sometimes seen in older literature. One Osm/L equals 1000 mOsm/L, so 1 mOsm/L is one thousandth of an Osm/L.

Osmolarity and osmolality refer to related but distinct concepts. Osmolarity (Osm/L) is the osmoles per liter of solution; osmolality (Osm/kg) is the osmoles per kilogram of solvent. In dilute, dilute solutions and for many physiological purposes, osmolarity and osmolality are numerically close, but they are not identical and can differ with temperature and solution density.

Calculation and examples: Osmolarity is approximately the sum of contributions from all solutes, using the van’t Hoff factor i to account for dissociation. For example, NaCl dissociates into two particles (i ≈ 2), so a 0.15 M NaCl solution has about 0.30 Osm/L or 300 mOsm/L. Glucose, which does not dissociate (i ≈ 1), at 0.15 M is about 150 mOsm/L. Solutes that partially dissociate or associate can alter the effective osmolality.

Measurement and use: Osmolar concentration can be measured with an osmometer, often employing freezing point depression or vapor pressure methods. Clinically, the osmolality is typically reported in mOsm/kg and plasma osmolarity is about 275–295 mOsm/L. The osmolar gap—difference between measured osmolality and calculated osmolarity from routine solutes—helps detect unmeasured solutes such as alcohols or ketones.