In the dilute ideal particle-count description, osmolarity is the total amount of substance of independently dissolved solute particles per volume of chemical solution: . It includes both membrane-permeating and retained solutes; specifying a particular membrane is unnecessary to count particles. A neutral undissociated solute contributes one particle per molecule, whereas separated ions are counted individually. Common units are or , with one osmole denoting one mole of the counted particles. Osmolality uses mass of solvent instead of volume of chemical solution; the two numerical values should not be identified without the appropriate mass-to-volume conversion.[ref]
Osmosis, Cell Membranes-Structure and Transport, BYU-Idaho. Cell Membranes—Structure and Transport, Osmosis: paragraphs beginning Earlier this semester and Osmolarity is a useful term; total particle counting
This ideal counting definition states its scope: interactions can require a thermodynamic activity description of osmotic pressure. Tonicity asks a different question, namely how that surrounding chemical solution changes a specified biological cell's volume through its particular cell membrane.
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Osmotic concentration refers to the measure of solute concentration in a solution, typically expressed in terms of osmoles per liter (osmol/L). It describes the total concentration of all solute particles in a solution that contribute to the osmotic pressure, which is the pressure required to prevent the flow of solvent molecules across a semipermeable membrane. In biological contexts, osmotic concentration is important for understanding how cells interact with their environment.