# 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).

### References

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.