How are osmolarity and osmolality different?

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    refers to concentration in terms of moles per litre of solution whereas osmolality refers to concentration in terms of moles per kilogram of . For dilute 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 of the solution is
    1.005 g/mL.
    Osmolarity
    0.850 g NaCl ##(1 mol NaCl)/(58.44 g NaCl)## = 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 ##(2 Osm)/(1 mol NaCl)## = 0.0291 Osm
    Osmolarity = osmoles/litres = ##(0.0291 Osm)/(0.100 L)## = 0.291 Osm/L or
    291 mOsm/L
    Osmolality
    Mass of solution = 100.0 mL ##(1.005 g)/(1 mL)## = 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

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