milliosmoles

Milliosmoles, abbreviated as mOsm, are a unit used to express osmotic concentration—the number of osmotically active particles in a solution. One osmole (Osm) equals Avogadro's number of particles that contribute to osmotic pressure, and one milliosmole is 1/1000 of that amount. Milliosmoles are commonly reported as milliosmolarity (mOsm/L) or, in some medical contexts, as milliosmolality (mOsm/kg), though the latter can be confusing because osmolarity and osmolality use different reference quantities (volume versus mass of solvent).

Osmolarity and osmolality are related but distinct concepts. Osmolarity measures osmoles per liter of solution, while osmolality measures osmoles per kilogram of solvent. In most dilute aqueous solutions at room temperature, 1 mOsm/L is approximately equal to 1 mOsm/kg, but small differences can arise with solute density or temperature. The total osmolarity is the sum of contributions from all solutes, and for solutes that dissociate, such as salts, the effective particle count is multiplied by the van't Hoff factor (i): Osmolarity ≈ Σ c_i × i, where c_i is the molar concentration.

Typical values provide practical context. Normal human serum osmolality is about 275–295 mOsm/kg. Isotonic saline (0.9% NaCl) has an osmolarity around 308 mOsm/L. Milliosmoles are widely used in physiology, medicine, and pharmaceutical formulation to ensure solutions are compatible with biological tissues and to avoid osmotic damage.

Note that mOsm is not an SI unit; it is a practical unit used specifically for osmotic concentration, with conventions varying by field and measurement method.