Difference Between Osmolality and Osmolarity

Chemistry is a vast domain to learn and the necessary terms can often be confusing with each other. Although the naming can be identical for many terms, their definitions and purposes are completely opposite to each other. Two such terms are Osmolality and Osmolarity that sound exceptionally similar but practically vary in properties.

Osmolality vs Osmolarity

The main difference between Osmolality and Osmolarity is that Osmolality is the number of solute particles per weight of solvent in kilograms while Osmolarity is the number of solute particles per volume of solution in liters. However, the unit that measures both Osmolality and Osmolarity is Osmoles or Osm.

Osmolality vs Osmoloarity

Osmolality is defined as the number of osmoles of solute per kilogram of solvent. Osmolality is generally calculated when the conditions include varying pressure and temperature. Since neither temperature nor pressure has an effect on the number of solutes or weight of solvent, osmolality can be calculated in all conditions.

Osmolarity is defined as the number of osmoles of solute per liter of a solution. Osmolarity is only calculated when the conditions include constant pressure and temperature. Although the number of solutes is constant, both temperature and pressure can change the volume of the solution. Thus, osmolarity can be calculated in only favorable conditions.

Comparison Table Between Osmolality and Osmolarity

Parameters of ComparisonOsmolalityOsmolarity
Formula UsedOsmolality = Osmoles/(One kilogram of solution)Osmolarity = Osmoles/(One Litre of solution)
Unit of MeasurementOsm/KgOsm/L
Temperature and Pressure DependencyDoes not depend on either temperature or pressure.Depends on both temperature and pressure.=
ConvenienceEasy to calculate and practically convenient to use because the solvent is constant. Even though it is easy to calculate, it is difficult to determine the volume of the solution as it may vary.

What is Osmolality?

Osmolality scientifically refers to the osmotic pressure of a solution regarding the solution mass. It is defined as the number of osmoles of solute per kilogram of solvent. The osmoles are defined as the total number of moles of particles present in that particular solution. These solute particles can be atoms, ions, and molecules, etc. For example, if you put sugar in your coffee, sugar is the solute and coffee is the solvent.

The instrument used to practically measure osmolality is Osmometer. There are various types of osmometer instruments available to calculate the osmolality of samples including freezing point osmometers, vapor pressure osmometers, etc. Osmometry is a crucial technology in clinical studies. The osmolality of body fluids is usually measured by freezing-point depression osmometry. The measurement of osmolality is also useful while diagnosing sodium disorders, potassium disorders, dehydration, poisoning, adrenal inconsistencies, neurological injuries, etc.

Based on their osmolality, the solutions are divided into three types:

  • Hyperosmotic: When osmolality is greater than that of the reference solution. For example, seawater.
  • Hypoosmotic: When osmolality is less than that of the reference solution. For example, distilled water.
  • Isosmotic: When osmolality is practically equal to the reference solution. For example, Boric Acid.

What is Osmolarity?

Osmolarity is scientifically the measure of solute concentration. It is defined as the number of osmoles of solute per liter of solution. Osmolarity is also known as Osmotic Concentration. Just like osmolality, the solute particles can be ions, atoms or molecules, etc. For example, Osmolarity leads to the measurement of the osmotic pressure of that solution and helps to study the solvent’s diffusion through a semi-permeable membrane. This process practically separates two solutions of different osmolarity.

Osmolarity decides the hydration status of the human body. It is important because if the osmolarity of the surroundings of the cell is different from the cell itself, the cell cannot survive. These cells can only survive if the water levels are balanced. Hence, osmolarity is necessary for the process of osmosis so cells can burst if they reach excessive water levels, and compress if they lose too much.

When comparing two solutions with different =osmolarities, three terms are used to define such distinct solutions:

  • Hyperosmotic: The solution with higher osmolarity is called hyperosmotic.
  • Hypoosmotic: The solution with lower osmolarity is called hypoosmotic.
  • Iso-osmotic: If two solutions have practically equal osmolarity, they are said to be iso-osmotic. For example, 0.9 % normal saline is iso-osmotic with tears.

Main Differences Between Osmolality and Osmolarity

  1. In osmolality, the osmoles are calculated with respect to the weight of solvent while in osmolarity the osmoles are calculated with respect to the volume of solution.
  2. Osmolality is generally easier to calculate than osmolarity because the solvent remains constant under all conditions. Hence, osmolality is also mostly used in clinical studies.
  3. Osmolality is used to determine medical conditions like dehydration and diabetes and Osmolarity is used to determine the concentration of dissolved particles in urine samples.
  4. Osmolality deals with the number of osmoles in a fluid. On the other hand, osmolarity deals with the concentration of an osmotic solution.
  5. Osmolality is always more accurate and less practical because everything remains constant while Osmolarity is more practical and less accurate as it varies with temperature.


Osmolality and osmolarity are the measurements of the solute concentration of a solution. Practically, there is little to no difference between the absolute values of the two measurements. Hence, both osmolality and osmolarity are mostly used interchangeably, even though they define different units of measurement.

In scientific experiments, osmolality is preferred over osmolarity due to colligative properties such as osmotic pressure, vapor pressure, and freezing point depression, etc., because these properties do not affect the mass of either solute or the solvent. As a result, the observations are made are very accurate. Not just that, osmolality is also independent of pressure and temperature. Hence, it is a favorable method. Also for dilute solutions like adulterated milk, detergents, etc., the osmolality and osmolarity are numerically the same values.

Osmolality can also also be used to calculate osmolarity. This makes the experimental calculation of the osmolarity irrelevant as there is no need to do so. Even though osmolality and osmolarity have their differences, they are mostly similar. For a regular study or experiment that is not directly based on either of the two, there is simply no difference between the two terms. This is the major reason that these two terms are often confused with each other.


  1. https://www.sciencedirect.com/science/article/pii/S0009912005000676
  2. https://www.sciencedirect.com/science/article/pii/S0022354915398920

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