= Osmotic concentration
{title2=$c_{\mathrm{osm}}$}
{wiki}
= Osmolarity
{synonym}
In the dilute ideal particle-count description, <osmolarity> is the total <amount of substance> of independently dissolved <solute> particles per <volume> of <chemical solution>: $c_{\mathrm{osm}}=\sum_j n_j/V$. 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 $\mathrm{osmol\,L^{-1}}$ or $\mathrm{mOsm\,L^{-1}}$, 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.<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]{ref}
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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