The Foundations of Chemistry

BOILING POINT ELEVATION

Recall that the boiling point of a liquid is the temperature at which its vapor pressure
equals the applied pressure on its surface (see Section 13-8). For liquids in open containers,
this is atmospheric pressure. We have seen that the vapor pressure of a solvent at a given
temperature is lowered by the presence in it of a nonvolatilesolute. Such a solution must
be heated to a higher temperature than the pure solvent to cause the vapor pressure of
the solvent to equal atmospheric pressure (Figure 14-14). In accord with Raoult’s Law,
the elevation of the boiling point of a solvent caused by the presence of a nonvolatile,
nonionized solute is proportional to the number of moles of solute dissolved in a given
mass of solvent. Mathematically, this is expressed as

TbKbm

The term Tbrepresents the elevation of the boiling point of the solvent, that is, the
boiling point of the solution minus the boiling point of the pure solvent. The mis the
molality of the solute, and Kbis a proportionality constant called the molal boiling point
elevation constant.This constant is different for different solvents and does not depend
on the solute (Table 14-2).

Kbcorresponds to the change in boiling point produced by a one-molal idealsolu-

tion of a nonvolatile nonelectrolyte. The units of Kbare °C/m.

14-11

When the solute is nonvolatile, only

the solventdistills from the solution.

14-11 Boiling Point Elevation 563

TbTb(soln)Tb(solvent). The boiling

points of solutions that contain

nonvolatile solutes are always higher

than the boiling points of the pure

solvents. So Tbis always positive.

Figure 14-14 Because a nonvolatilesolute lowers the vapor pressure of a solvent, the
boiling point of a solution is higher and the freezing point lower than the corresponding
points for the pure solvent. The magnitude of the boiling point elevation, Tb, is less than
the magnitude of the freezing point depression, Tf.

Solid

Liquid

Gas

Psolvent

at bp of

pure solvent

Sol

uti

P on

ure

sol

ven

t

760

Vapor pressure of solvent (torr)

Tf

solution

Tb

solution

Tf

pure

solvent

Tb

pure

solvent

Tf Tb

Temperature (°C)