Whereas osmolality (with an “ℓ”) is a measure of the osmoles (Osm) of solute per kilogram of solvent (osmol/kg or Osm/kg), osmolarity (with an “r”) is defined as the number of osmoles of solute per liter (L) of solution (osmol/L or Osm/L). 1 Measured osmolality (MO)