Page 62 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

Phase and Chemical Equilibria 47

Schematic diagram of an ideal solution. The dashed lines are the partial
Fig. 6.2
pressures of components A and B. The drawn line is the total pressure.

An example of an ideal solution is a mixture of benzene and toluene
(see Fig. 6.2).

Note: Each partial pressure is x A P or x B P and the total pressure
∗
∗
A B
is the sum of the two.
It can be shown that when a substance obeys Raoult’s Law, the
chemical potential becomes

o
µ A = µ + RT ln x A (6.12)
A
and thus a A = x A [see also Eq. (5.33)].

6.2.2. Ideal Dilute Solutions. Henry’s Law
Ideal solutions are not very common. Most solutions will show deviations
from ideality. However, when solutions are very dilute, the solvent will obey
Raoult’s Law (as noted) and the solute will obey Henry’s Law.
Henry’s Law states that P B = x B K B,where K B is a constant, Henry’s
Law constant, generally not equal to the pressure of pure B, P .Henry’s
∗
B
Law can also be defined in terms of the molarity P B = m B K H.A solution
in which the solvent obeys Raoult’s Law and the solute obeys Henry’s Law