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 mol-
ecular dissociation.
n = nu mber 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 mea-
surement 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.
Freezing Point/Melting Point: The temperature at which the liquid
and solid phases of a substance will remain together in equilibrium.
Freezing-Point Depression: When a solute is added to a solvent, the
freezing point of the solvent is lowered. In aqueous solutions, one
Foreword
xxi
Osmolality = ønC =
osmol
kg H
2
O
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