# How are osmolarity and osmolality different?

Jan 24, 2014

Osmolarity refers to concentration in terms of moles per litre of solution, whereas osmolality refers to concentration in terms of moles per kilogram of solvent. For dilute solutions, however, they are numerically almost the same.

Osmolarity is the number of osmoles per litre of solution.
Osmolality is the number of osmoles per kilogram of solvent.

EXAMPLE

Calculate the osmolarity and the osmolality of a solution of
0.850 g of NaCl in 100.0 mL of water. The density of the solution is
1.005 g/mL.

Osmolarity

0.850 g NaCl × $\left(1 \text{ mol NaCl")/(58.44" g NaCl}\right)$ = 0.0145 mol NaCl.

In solution, 1 mol NaCl forms 1 mol Na⁺ + 1 mol Cl⁻= 2 mol particles, so 1 mol NaCl = 2 Osm

0.0145 mol NaCl × $\left(2 \text{ Osm")/(1" mol NaCl}\right)$ = 0.0291 Osm

Osmolarity = osmoles/litres = $\left(0.0291 \text{ Osm")/(0.100" L}\right)$ = 0.291 Osm/L or
291 mOsm/L

Osmolality

Mass of solution = 100.0 mL × $\left(1.005 \text{ g")/(1" mL}\right)$ = 100.5 g

Mass of solvent = (100.5 – 0.850) g = 99.6 g

Osmolality = "osmoles"/"kg solvent" = (0.0291" Osm")/(0.0996" kg") = 0.292 Osm/kg or 292 mOsm/kg