There are a number of unique units of measurement used in chemistry. A well-known example of this is pH, which is a scale used to identify how acidic or basic a substance is. However, some lesser-known units of measurement are also important in some fields. One that plays a big part in the medicine and other specialized fields is osmolarity, which is also known as osmotic concentration; as the name implies, it's a measurement of the concentration of a solute (in units known as osmoles) within a certain amount of a solution.

The osmolarity of a solution is a measure of how concentrated the solute is within one liter of the solution. This is measured in a unit known as osmoles (Osm), with osmotic concentration being written as osmoles per liter (Osm/L). In some instances, you will also see osmotic concentration referred to in terms of millimoles per liter (mmol/L) as well. As the amount of water or solvent decreases, the osmotic concentration of the solute increases. Likewise, an increase in the amount of solvent within a solution will decrease the osmotic concentration of the solute.

The osmole is a non-SI unit of measurement, meaning that it hasn't been standardized as part of the International System of Units. It is the measure of the number of moles of a solute that contribute to the osmotic pressure of a chemical solution. Part of the reason that it has not been standardized is that the osmole isn't used exclusively for this measurement; in situations where the osmotic pressure of the solution are unimportant, millimoles per liter may be used to measure the moles of solute within a solution instead.

Osmolarity is closely tied with osmosis, which is why the osmole is used to measure the osmotic concentration of a solution. The osmotic pressure of a solution refers to how much pressure is required to create an equilibrium by moving a solution through a semipermeable membrane. The osmolarity of a solution refers to the concentration of osmoles required to create this equilibrium, with the osmoles increasing osmotic pressure as concentrations increase.

Osmolarity is sometimes referred to alongside osmolality, which is a related measurement that also deals with the concentration of osmoles within a solution. The primary difference between the two is that while osmolarity measures the number of osmoles in a liter of a solution, osmolarity measures the number of osmoles per kilogram (Osm/kg) of solvent. As with osmolarity, you may also see osmolality written in terms of millimoles per kilogram (mmol/kg) in some instances.