Colligative Properties of Solutions

GENERAL CHEMISTRY 2
SVDPA eCLASSROOM
COLLIGATIVE
PROPERTIES OF
NONELECTROLYTES
AND ELECTROLYTE
SOLUTIONS
2
Colligative Properties
Colligative properties are properties of a solution that depend only on the

number and not on the identity of the solute particles. Thus, these depend on the
collective effect of the concentration of solute particles present in an ideal solution.
Because of their direct relationship to the number of solute particles, the colligative
properties are very useful for characterizing the nature of a solute after it is dissolved
in a solvent and for determining the molar masses of substances.
The Different Colligative Properties of Solution
Colligative properties include the following:

(1) vapor pressure lowering;
(2) boiling point elevation; and
(3) freezing point depression.
Effect of solute concentration on the colligative

properties of solutions
The concentration or amount of nonvolatile solute (i.e., a solute that does not
have a vapor pressure of its own) in the solution has an effect on the colligative
properties of solutions. The effect would depend on the ratio of the number of particles
of solute and solvent in the solution and not on the identity of the solute. However, it is
necessary to take into account whether the solute is an electrolyte or a nonelectrolyte.
Effects of electrolyte and nonelectrolyte on

colligative properties solutions.
Vapor Pressure Lowering

Vapor pressure is a direct measure of escaping tendency of molecules. A
pure liquid (solvent) in a closed container will establish equilibrium with its
vapor. And when that equilibrium is reached, the pressure exerted by the vapor
3
is called the vapor pressure. A substance that has no measurable vapor pressure
is nonvolatile, while one that exhibits a vapor pressure is volatile.
When a liquid evaporates easily, it will have a large number of its

molecules in the gas phase resulting to a high vapor pressure. The picture (Fig. 1)
in the left shows a surface entirely occupied by liquid molecules, some of which
evaporated and form a vapor. On the right, a nonvolatile solute like salt or sugar
has been dissolved into the solvent, having the effect of diluting the water. The
addition of a nonvolatile solute resulted to a lowering of the vapor pressure of
the solvent. The lowering of the vapor pressure depends on the number of
solute particles that have been dissolved. The chemical nature of the solute is
not considered because vapor pressure is merely a physical property of the
solvent and does not undergo a chemical reaction with the solvent and does not
itself escape into the gas phase.
It is important to note that the reduction in the vapor pressure of a

solution of this example is directly proportional to the fraction of the volatile
molecules in the liquid, which is the mole fraction of the solvent. This reduced
vapor pressure can be determined using Raoult’s Law (1886).
4
Recall from the definition of mole fraction that in a two component

solution (a solvent and a single solute), X solvent = 1 – X solute.
While the chemical nature of the solute is not a factor to consider, it is

important to take into consideration whether the solute is an electrolyte or
nonelectrolyte. Ionic compounds like sodium chloride, NaCl, are strong
electrolytes that dissociate into ions when they dissolve in solution results in a
larger number of dissolved particles. Consider two different solutions of equal
concentration: one is made from ionic compound NaCl, while the other is made
from the molecular compound glucose (C6H12O6). The equations below show
what happens when these solutions dissolve:
5
The sodium chloride, NaCl dissociates into 2 ions, while glucose does not
dissociate. Thus, equal concentrations of each solution will result in twice as
many dissolved particles as in the case of NaCl. The vapor pressure of the
solvent in NaCl solution (electrolyte) will be lowered twice as much as that of the
solvent in the glucose (nonelectrolyte) solution. Since the surface now of salt
solution is covered by more solute particles,there is less room for solvent
molecules to evaporate lowering the vapor pressure of the solvent, water.
1. Boiling Point Elevation
The addition of a nonvolatile solute lowers the vapor pressure of the

solution; consequently the temperature must be raised to restore the vapor
pressure of the solution to the value conforming to the pure solvent.
Specifically, the temperature at which the vapor pressure is 1 atm will be higher
than the normal boiling point by an amount known as the boiling point
elevation.
Picture below shows the phase diagram of a solution and the effect that
the lowered vapor pressure has on the boiling point of the solution compared to
the solvent. In this case the sucrose solution has a higher boiling point than the
pure solvent. Since the vapor of the solution is lower, more heat must be
supplied to the solution to bring its vapor pressure up to the pressure of the
external atmosphere. The boiling point elevation is the difference in
temperature between the boiling point of the pure solvent and that of the
solution.
6
For dilute solution the elevation of the boiling point is directly

proportional to the molal concentration of the solute:
The molal boiling point elevation constant, Kb, has a specific value
depending on the identity of the solvent.
7
2. Freezing Point Elevation
The freezing point of a substance is the temperature at which the solid and
liquid forms can coexist indefinitely, at equilibrium. Under these conditions molecules
pass between the 2 phases at equal rates because their escaping tendencies from the
two phases are identical.
Picture below shows the phase diagram for a pure solvent and how it changes
when a solute is added to it. The solute lowers the vapor pressure of the solvent
resulting in a lower freezing point for the solution compared to the pure solvent. The
freezing point depression is the difference in temperature between the freezing point
of a pure solvent and that of a solution. On the graph, Tf represents the freezing point
depression.
At a given temperature, if a substance is added to a solvent like water, the

solute-solvent interactions prevent the solvent from going into the solid phase,
requiring the temperature to decrease further before the solution will solidify.
Meaning, more energy must be removed from the solution in order to freeze it and the
freezing point of the solution is power than that of the pure solvent.
The magnitude of the freezing point depression is directly proportional to the

molality of the solution. Thus:
8
Where:
Kf – is the molal freezing - point depression constant., a constant that is equal to the
change in the freezing-point for a 1 molal solution of a nonvolatile molecular solute
Tf – freezing point depression
M – molality of solute

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GENERAL CHEMISTRY 2

Colligative properties are properties of a solution that depend only on the

The Different Colligative Properties of Solution

Colligative properties include the following:

Effect of solute concentration on the colligative

Effects of electrolyte and nonelectrolyte on

Vapor Pressure Lowering

When a liquid evaporates easily, it will have a large number of its

It is important to note that the reduction in the vapor pressure of a

Recall from the definition of mole fraction that in a two component

While the chemical nature of the solute is not a factor to consider, it is

1. Boiling Point Elevation

The addition of a nonvolatile solute lowers the vapor pressure of the

For dilute solution the elevation of the boiling point is directly

2. Freezing Point Elevation

At a given temperature, if a substance is added to a solvent like water, the

The magnitude of the freezing point depression is directly proportional to the

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