Molarity and osmolarity are two distinct concepts. Molarity (M) is the number of moles of solute per liter of solution. The unit of molarity is the mole (mol). Osmolarity (Osm/L) is the total concentration of all solutes in the solution. The unit of osmolarity is the osmol (osm). Osmolarity can be used to predict whether water will move from one side of a semipermeable membrane to the other.