Osmolality (ℓ) is the number of osmoles (Osm) of solute per kilogram of solvent (mOsmol/kg), it is measured using an osmometer. This value is not affected by temperature and pressure.
This differs from Osmolarity (r) which is the number of osmoles of solute per litre of solution (mOsmol/L) and is affected by changes in water content, temperature and pressure.
The difference between the actual osmolality and the calculated osmolality is known as the osmotic gap 
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