Colligative Properties of Solutions and Molality
The way solutions behave…
 are affected by the presence of dissolved solutes, and
 depend on the quantity of dissolved particles present, not the identity
Properties that depend on solute concentration, but not identity, are called colligative properties.
Calculations involving these properties will use concentration in molality, m.
Molality (m) is the number of moles of solute dissolved in one kilogram of solvent.
Three important colligative properties of solutions are..
 freezing-point depression
 boiling-point elevation
1. Freezing-point depression
A 1-molal solution of any nonelectrolyte solute in water will lower the freezing point 1.86 ºC. This value is
called the molal freezing point constant (Kf). Each solvent has its own distinct Kf.
The difference between the freezing points of the pure solvent and a solution of a nonelectrolyte in that
solvent is called the freezing-point depression, tf.
Can be calculated using…
tf = iKfm
where Kf is in ºC, m is molality, and i=1 (a dissociation value)
2. Boiling-point elevation
A 1-molal solution of any nonelectrolyte solute in water will raise the boiling point 0.51 º C. Thus, the molal
boiling point constant (Kb) is 0.51 ºC.
The difference between the boiling points of the pure solvent and a solution of a nonelectrolyte in that
solvent is called the boiling-point elevation, tb.
Can be calculated using…
tb = iKbm
where Kf is in ºC, m is molality, and i=1 (a dissociation value)
The (i) value is based on the degree of dissociation of the solute. Nonelectrolytes do not dissociate. The i
value is always 1 in this case. For electrolytes, like acids and ionic compounds, the dissociation depends on
the compound.
Compound
Electrolyte/Nonelectrolyte
i Value
Examples:
C6H12O6 (no dissociation)
non electrolyte
1
NaCl
electrolyte
2
CaCl2
electrolyte
3
Recall, dissociation equations…
NaCl → Na+ + ClCaCl2 → Ca2+ + 2Cl-
(dissociates into two ions)
(dissociates into three ions)