Osmolarity is expressed in which units?

• A. osm/L
• B. osm/kg
• C. mol/L
• D. g/L

Answer: (A)

Osmolarity measures how many dissolved particles are present per liter of solution. It is expressed in osmoles per liter (osm/L), often written as mOsm/L in practice. This reflects the total number of particles, not just the amount of solute, and it accounts for how electrolytes dissociate into multiple particles (for example, NaCl at 1 M contributes about 2 osmoles per liter because it splits into Na+ and Cl−).

Osmolality is a related concept but uses kilograms of solvent (osmol/kg), not liters, and can differ from osmolarity when volumes change with temperature or pressure. The other units—mol/L and g/L—represent molarity (moles per liter) and mass concentration, respectively, not the total particle concentration that osmolarity describes.