How are osmolarity and osmolality different?
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.
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
0.850 g 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 ×
Osmolarity = osmoles/litres =
Mass of solution = 100.0 mL ×
Mass of solvent = (100.5 – 0.850) g = 99.6 g