http://www.ck12.org Chapter 16. Solutions
the solvent. Nonvolatile means that the solute itself has little tendency to evaporate. Because some of the surface is
now occupied by solute particles, there is less room for solvent molecules. This results in less solvent being able to
evaporate. The addition of a nonvolatile solute results in a lowering of the vapor pressure of the solvent.
FIGURE 16.10
The solution on the right has had some
of its solvent particles replaced by solute
particles. Since the solute particles do
not evaporate, the vapor pressure of the
solution is lower than that of the pure
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 important because the vapor pressure is merely a physical property of the solvent. The
only requirement is that the solute does not undergo a chemical reaction with the solvent and does not itself escape
into the gaseous phase.
While the chemical nature of the solute is not a factor, it is necessary to take into account whether the solute is an
electrolyte or a nonelectrolyte. Recall that ionic compounds are strong electrolytes that dissociate into ions when
they dissolve. This results in a larger number of dissolved particles. For example, consider two different solutions
of equal concentration. One is made from the ionic compound sodium chloride, while the other is made from the
molecular compound glucose. The following equations show what happens when these solutes dissolve.
NaCl(s)→Na+(aq)+Cl−(aq)2 dissolved particles
C 6 H 12 O 6 (s)→C 6 H 12 O 6 (aq)1 dissolved particleThe sodium chloride dissociates into two ions, while the glucose does not dissociate. Therefore, equal concentrations
of each solution will result in twice as many dissolved particles in the case of the sodium chloride. The vapor pressure
of the solvent in the sodium chloride solution will be lowered twice as much as that of the solvent in the glucose
solution.
Freezing Point Depression
The image below (Figure16.11) 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. Thefreezing point depressionis the difference in temperature between the freezing
point of a pure solvent and that of a solution. On the graph, the freezing point depression is represented by∆Tf.
When a pure solvent freezes, its particles become more ordered as the intermolecular forces that operate between
the molecules fix each molecule in place. In the case of water, hydrogen bonds become more rigid and generate
the hexagonally shaped network of molecules that characterizes the structure of ice. By dissolving a solute into the
liquid solvent, this ordering process is disrupted. As a result, more energy must be removed from the solution in
order to freeze it, and the freezing point of the solution is lower than that of the pure solvent.
In calculations involving colligative properties, the concentration of the solute will be measured by its molality.
Molality is used rather than molarity because the molality of a solution is independent of its temperature. Recall that
molality is defined as the moles of solute divided by the kilograms of solvent. Molarity is the moles of solute divided
by the volume of the solution. While mass does not depend on temperature, the volume of the solution increases