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Difference Between Osmolality and Osmolarity

chemical-lab-pd2Osmolality vs Osmolarity

Osmolality and osmolarity are units of measurement. Osmolality is the number of osmoles of solute in a kilogram of solvent, while osmolarity is the number of osmoles of solute in a litre of solution. An osmole is one mole of any non-dissociable substance. It will contain 6.02 x 1023 particles.

Osmolarity is the concentration of an osmotic solution. This is usually measured in osmoles. Osmolarity is also used to determine certain medical conditions, like the dissolved particles in urine. The volume of a solution will change with the addition of solutes, and also with any change in the temperature or pressure. Therefore, osmolarity is sometimes difficult to determine.

Osmolality deals with the concentration of the particles that is dissolved in a fluid. In medical science, osmolality is used to determine several conditions like diabetes, dehydration and shock. For the detection of these conditions, the osmolality of the serum is checked, and is known as plasma osmolality. The concentration of the substances like chloride, sodium, potassium, glucose and urea are calculated.

The volume of a solvent remains the same regardless of any changes in pressure or temperature, so it is relatively easier to determine the osmolality. Owing to this fact, osmolality is the common method of measurement in Osmometry.

Osmolality measures the number of particles in the unit weight of a solvent, and is independent of the shape, size or weight of the particles. The properties based on which the particle concentration is measured are known as Colligative properties. The properties are vapour pressure depression, freezing point depression, boiling point elevation, and osmotic pressure.

Usually osmolarity is expressed as Osm/L, and osmolality as Osm/Kg. To measure osmolality an instrument known as an osmometer is used, and it works by using the method of freezing point depression.

The difference between the calculated osmolarity and measured osmolality is known as the osmolar gap. The values are presented in different units, and this is due to the difference in the method of the calculation and measuring. The abbreviation for measured osmolality is MO, and that for calculated osmolarity is CO. The osmolar gap is represented as OG. If the concentration of solutes in the given fluid is very low, then osmolality and osmolarity are considered to be equivalent.


1. If osmolality is the number of osmoles of solute in a kilogram of solvent, then osmolarity is the number of osmoles of solute in a litre of solution.
2. Osmolarity deals with the concentration of an osmotic solution, while osmolality deals with the concentration of particles in a fluid.
3. It is easier to determine the osmolality than the osmolarity.
4. Osmolarity is expressed as Osm/L, and osmolality is expressed as Osm/Kg.
5. Osmolality is used to determine medical conditions like diabetes, shock and dehydration, while osmolarity is used for the detection of the concentration of dissolved particles in urine.
6. Osmolality is the commonly used method of measurement in Osmometry.
7. When the concentration of solutes is very low, the osmolality and osmolarity are similar.

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  1. First paragraph contains an error ”

    “An osmole is one mole of any non-dissociable substance. It will contain 6.02 x 1023 particles.”

    Should say 6.02 x 10 raised to the 23rd power

  2. I found the page useful. Thanks.

  3. I really like this site. Kudos

  4. good concept… I just want to elaborate practical application mentioned in article..

    so qn arises- why osmolaRITY for urine and osmolaLITY for blood ?

    answer: because blood is in dynamic condition and urine is waste product and static and in our control…

    what I mean is that in case of urine analysis we can maintain constant temperature, pressure and obviously we are not goin to add any extra solute in sample…so we can measure osmolaRITY..

    In case of blood its under dynamic condition… volume and pressure are changing solutes are added n removed always… once we take a sample and analyse osmolarity 1, on sampling next time we get a value of osmolarity 2… both value are not pure indicator of solute as they are biased by temperature, pressure changes and other solutes which are not our focus…and so is the ERROR.. so to standardize these error we find other modality that is osmolality twhich is not influenced by these factors…

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