millimole per kilogram

The millimole per kilogram belongs to samples whose density cannot be trusted or whose temperature will change between collection and analysis. Seawater is the standard case: reference seawater carries about 469 mmol of sodium, 53 of magnesium and 10 of calcium in each kilogram, figures that hold whether the sample sits at 2 °C at depth or 25 °C on the bench, which is exactly why ocean and hydrothermal chemists report on a mass basis.

One honesty note on that example: ocean data is quoted per kilogram of seawater, solution rather than solvent, so it is strictly a different quantity from the molality this page defines. In dilute fresh water the distinction vanishes; in seawater, where 35 g of every kilogram is salt, the two bases differ by about 3.5 percent, and careful geochemical work states which it means.

The clinical cousin is osmolality. A freezing-point osmometer measures the same colligative depression this site's freezing-point solver computes, then reports it as milliosmoles per kilogram of water: healthy serum sits near 285 mOsm/kg, and the gap between measured and calculated osmolality is a standard screen for unmeasured solutes such as ingested alcohols. The per-kilogram basis is what makes those numbers comparable across patients, instruments and temperatures, for the same reason it works at the bottom of the ocean.