Page 86 - Basic physical chemistry for the atmospheric sciences

72 Basic physical chemistry

l
Similar y , if the solute (B) is volatile
=
Ps "1spg (4.5)
A solution that obeys Raoult's law exactly is called an ideal solution.
Solutions that are associated with either exothermic or endothermic
reactions are not ideal solutions. Raoult's law is most accurate when
used to describe components of a solution that are present in high
concentrations. At low concentrations there are often significant de­
partures from Raoult's law. At very low concentrations the vapor
pressure of a solute is given by Henry's law.
A liquid boils when its saturated vapor pressure is the same as the
e
atmospheric pressur . Since a nonvolatile solute will lower the vapor
pressure of a solution, a higher temperature will be required to cause
the solution to boil. The increase in the boiling point of a solution
(LlT8) above that of the pure solvent is approximately proportional to
the molality (m)
(4.6)

where Kb, the molal boiling-point-elevation constant, depends on the
solvent. For water Kb = 0.52°C m - 1 • Note that the elevation of the
boiling point is proportional to the number of solute particles present
in a given amount of solution.
The freezing point of a substance is the temperature at which the
saturated vapor pressures of the solid and liquid phases are the same.
Since solutes are not normally soluble in the solid phase of the solvent,
the vapor pressure of the solid is unaffected by the solute. On the
other hand, if the solute is nonvolatile, the vapor pressure of the
solution is reduced. Consequently, the temperature at which the solu­
tion and solid phase will have the same saturated vapor pressure (i. . ,
e
i
i
the freezing point) s reduced. The reduction n the freezing point of a
solution (LlTr ) is given approximately by
(4.7)
where Kc is called the molal f r eezing-point-depression constant. For
l
water Kr is . 86°C molality - 1 •

4.5 Aqueous solutions; electrolytes
The central role that water plays on Earth is due not only to its great
abundance but also to its unique ability to dissolve (at least partially)
many substances. Consequently, aqueous solutions are extremely