The amount of solute is usually expressed in terms of moles, i.e., gram
molecular weights. One mole = 6.028 x 10
23
molecules (Avogadro's
number). One mole of glucose (180.2 g) and one mole of sodium chlo-
ride (58.4 g) each contain Avogadro's number of molecules.
Common units of concentration are:
• Molality: Moles of solute per kilogram of pure solvent.
• Osmolality: Osmols of solute particles per kilogram of pure sol-
vent. As noted above, most ionic solutes do not completely dis-
sociate. Osmolality is a unit of concentration that takes into
account the dissociative effect. Osmolality is usually expressed
in mOsm/kg H
2
O. One milliosmol (mOsm) is 10
-3
osmols.
Osmolality is defined as:
where:
ø = osmotic coefficient, which accounts for the degree of mole-
cular dissociation.
n = number of particles into which a molecule can dissociate.
C = molal concentration of the solution.
• Molarity: Moles of solute per liter of solution.
• Osmolarity: Osmols of solute particles per liter of solution.
Although molarity and osmolarity may be common units of
measurement in other branches of chemistry, they are not used in
osmometry because the ratio of solute to solution is not linear.
Molality and osmolality are linear, independent of the effect of
temperature and volume displaced by solute. A calculated con-
version between units of molality and molarity is complex and
generally unnecessary when the terms are properly understood.
xix
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