Chemistry - What is the difference in mosM and mOsm units, if there is any at all?

Solution 1:

I think the clinical chemistry notation has not been standardized but certainly it has been inspired by chemists.

The capital M in mosM emphasizes that it is milliosMolar. Chemists use capital M to denote molarity.

On the other hand, mosm indicates milliosmolal. Chemists use small m to symbolize molality.

This notation is not universal.

From this slightly dated article on Osmolality, which is behind pay-wall Osmolality, Harry F. Weisberg Laboratory Medicine, Volume 12, Issue 2, 1 February 1981, Pages 81–85.

Terminology and Units

In 1934, Gamble, et al.,11 introduced the term ”osmillimole” which has been successively transformed into mosM (milliosmolar), mosm (milliosmolal), the noncommittal mOsm, and the present-day mosmol (milliosmole). Raoult’s law states that the lowering of the vapor pressure (one of the four ”colligative” properties of dilute solutions) is proportional to the mole-fraction (more easily calculated from molality) of solute present in the mixture.

Solution 2:

Judging from the following table consisting of compiled notations presented in offline edition of The ACS Style Guide and AMA manual of style, section 13.12 Units of Measure:

$$ \begin{array}{ll} \hline \text{Unit of measure} & \text{Symbol} & \text{Ref.} \\ \hline \text{osmolar} & \text{osm (also osM, Osm)} & \text{[1, p. 191]} \\ \text{osmole} & \text{osm} & \text{[2, p. 635]} \\ \text{microosmole} & \text{µOsm} & \text{[2, p. 634]} \\ \text{milliosmole} & \text{mOsm} & \text{[2, p. 634]} \\ \hline \end{array} $$

symbols $\pu{Osm}$ and $\pu{osm}$ refer to the amount-of-substance of an osmotically active entity, and symbol $\pu{osM}$ refers to osmolar concentration suggesting the following relation:

$$\pu{1 osm L^-1} = \pu{1 Osm L^-1} = \pu{1 osM}$$

The letter “o” in $\pu{Osm}$ appears to be capitalized for the sake of better readability when used in conjunction with metric prefixes, and the letter “m” in in $\pu{osM}$ is likely capitalized to resemble analogy with the unit symbol for molarity (small capital letter “m”: ᴍ).


  1. The ACS Style Guide: Effective Communication of Scientific Information, 3rd ed.; Coghill, A. M., Garson, L. R., Eds.; American Chemical Society; Oxford University Press: Washington, DC; Oxford; New York, 2006. ISBN 978-0-8412-3999-9.
  2. AMA Manual of Style: A Guide for Authors and Editors, 11th ed.; American Medical Association, Ed.; Oxford University Press: New York, 2020. ISBN 978-0-19-024656-3.