mOsm

Molesolute unit called milliosmole (mOsm) is a unit used to express osmotic quantity in solutions. An osmole (Osm) is the amount of substance that contributes to osmotic pressure as if it contained one mole of particles. Therefore, 1 Osm equals 1000 mOsm. In practice, osmolarity is often reported as milliosmoles per liter (mOsm/L) and osmolality as milliosmoles per kilogram of water (mOsm/kg). In dilute biological fluids, osmolarity and osmolality are typically very close, so mOsm/L and mOsm/kg are used somewhat interchangeably in clinical contexts.

Calculation and factors: The osmolarity of a solution is the sum over solutes of i × C, where C is the molar concentration and i is the van’t Hoff factor, representing the number of particles produced per solute molecule. Non-dissociating solutes (like glucose or urea) have i ≈ 1, while electrolytes that dissociate increase i (e.g., NaCl with i ≈ 2; CaCl2 with i ≈ 3). The milliosmolar value is simply osmolarity (Osm/L) multiplied by 1000.

Typical values: Human plasma osmolarity is about 275–295 mOsm/L, corresponding to an osmolality of roughly 285–295 mOsm/kg. Examples: 0.15 M NaCl yields ~0.30 Osm/L (300 mOsm/L); a 5% glucose solution yields about 0.28 Osm/L (around 280 mOsm/L); CaCl2 at 0.1 M yields about 0.30 Osm/L depending on dissociation.

Measurement and uses: Osmolality can be measured by osmometry using freezing-point depression or vapor-pressure methods. Osmolality and osmolarity are fundamental in physiology and medicine for assessing hydration status, electrolyte balance, and the tonicity of body fluids and intravenous solutions.