Physical Chemistry , 1st ed.

liquids are immiscible, so the total pressure in equilibrium with the liquids is
simply the sum of the two equilibrium vapor pressures.*

7.5 Liquid/Gas Systems and Henry’s Law

Gases can dissolve in liquids. In fact, liquid/gas solutions are important to us.
One example is a soft drink, which has carbon dioxide gas dissolved in water.
Another example is the ocean, where the solubility of oxygen is crucial to fish
and other animal life, and the solubility of carbon dioxide is important for algae
and other plant life. In fact, the ability of the oceans to dissolve gases is largely
unknown but is thought to be a major factor in the weather conditions of the
troposphere (the layer of the atmosphere closest to the surface of the earth).
Liquid/gas solutions range between extremes. Hydrogen chloride gas, HCl,
is very soluble in water, making solutions of hydrochloric acid. By contrast, the
solubility of 1 bar of pure oxygen in water is only about 0.0013 M.
Since liquid/gas solutions are nonideal, Raoult’s law does not apply. This is
illustrated in Figure 7.19, in which the vapor pressure of some gaseous com-
ponent is plotted versus mole fraction. The figure shows a range of mole frac-
tion where Raoult’s law gives good predictions when compared to reality.
However, this region is concentrated at large values of mole fraction; for most
compositions, Raoult’s law disagrees with real measurements.
However, Figure 7.19 does show that in regions of low mole fraction, the
vapor pressure of the gas in the equilibrium vapor phase isproportional to
the mole fraction of the component. This proportionality is illustrated by an
approximately straight dotted line in the plot of pressure versus xiat low mole
fractions. Since the vapor pressure is proportional to the mole fraction, we can
write this mathematically as

pi xi

7.5 Liquid/Gas Systems and Henry’s Law 183

*Here we are assuming that both liquids are exposed to some space within the system
and can come to equilibrium with their vapor phases. In systems where a denser immisci-
ble liquid is completely covered by a less dense liquid, its vapor pressure will be suppressed.

p* 1

Pressure

0.5

xgas

0.0 1.0

Henry’s law behavior

Raoult’s law behavior

Real behavior

Region where

Henry’s law applies

Region where Raoult’s

law does not apply

Region

where Raoult’s

law applies

Figure 7.19 If a gas is one of the components, Raoult’s law does not hold at low mole frac-
tions of gas. However, there is a region of proportionality. This region can be described using
Henry’s law.