Osmolar

Osmolar is a term used in chemistry and physiology to describe the concentration of osmotically active particles in a solution. It refers to the amount of solute that contributes to osmotic pressure, expressed as osmoles per liter of solution (Osm/L). An osmole is the amount of solute that yields one mole of effective particles in solution; non-dissociating solutes contribute one osmole per mole, while dissociating solutes yield more osmoles per mole (for example, NaCl yields about 2 osmoles per mole in ideal conditions). Because of this, the osmolar concentration accounts for both the amount and the particle number of solutes.

Osmolarity and osmolality are related but distinct concepts. Osmolarity denotes osmoles per liter of solution (Osm/L) and is volume-based, whereas osmolality denotes osmoles per kilogram of solvent (Osm/kg) and is mass-based. In dilute biological solutions, osmolarity and osmolality are typically close in value, and the term osmolar is often used interchangeably with osmolarity in casual contexts. Typical human plasma osmolarity is around 275 to 295 mOsm/L.

Measurement and calculation are common approaches. Osmolarity can be calculated by summing the contributions of significant solutes, commonly approximated in clinical settings by 2[Na+] + [glucose]/18 + [BUN]/2.8 (all in milliequivalents or mg/dL as appropriate), yielding an estimated osmolality close to osmolarity. Direct measurement via osmometry uses physical properties such as freezing point depression or vapor pressure.

Clinically, osmolar concentration influences cell volume and fluid balance. Abnormal osmolarity—whether hypoosmolar or hyperosmolar states—can indicate or drive disorders such as hyponatremia, hyperglycemia, or dehydration. The term osmolar is thus central to discussions of osmosis, osmotic pressure, and related physiological processes.