Osmolality – measurement of number of osmoles of solute dissolved in each kilogram of water for a particular solution. A solution with 1 osmole of solute dissolved in 1kg of water has an osmolality of 1 osmole/kg (of water).
Osmolarity – measurement of number of osmoles of solute dissolved in each litre of a particular solution. A solution with 1 osmole of solute dissolved in water to make 1 litre of solution has an osmolarity of 1 osmole/L (of solution).
To determine osmotic pressure, osmolality is supposed to be used but for dilute solutions (e.g. body fluids) osmolarity can be used instead, as the difference is less than 1% and osmolarity is more readily measured.
Source: Guyton & Hall, Medical Physiology 11 ed.
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