What is Osmolarity?

••• MangoStar_Studio/iStock/GettyImages

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.

TL;DR (Too Long; Didn't Read)

Osmolarity is the measure of how many osmoles of a solute are present in 1 liter of a solution.

Osmotic Concentration

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.

What Is an Osmole?

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.

What Is Osmotic Pressure?

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 vs. Osmolality

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.


About the Author

Holding a BS in computer science and several years of experience building, repairing and maintaining computers and electronics, Jack Gerard has had a love of science and mathematics for years. When not working on writing projects as part of his 15+ year career, he also works as a programmer writing gaming and accessibility software.