Chemistry, Third edition

11 · SOLUTIONS AND SOLUBILITY

for example,

water

NH 3 (g)\===\NH 3 (aq)

followed by

NH 3 (aq) + H 2 O(I)\===\NH 4 (aq) + OH(aq)

Variation of the solubility of gases with the partial

pressure of the gas

How does the solubility of gases vary with the pressure of the dissolving gas? Because a

gas at high pressure will be ‘pushed’ more into the water than a gas at low pressure, we

predict that the greater the (partial) pressure of the dissolving gas the greater the concen-

tration of gas in solution (Fig. 11.5). This turns out to be the case for all gases:

The solubility of a gas increases with its partial pressure.

The data in Table 11.5 refer to the case when the partial pressure of the dissolving gas

is 1 atm (101 kPa). Notice that ammonia is the most soluble of all the gases listed.

Measurements of the partial pressure of a gas and its equilibrium concentration

in solution may be plotted graphically. Some gases (Fig. 11.6) give straight line plots

and obey the equation

cKHp

wherecis the concentration of gas in solution (in mol dm^3 ),pis the partial pres-

sure of the dissolving gas (in atm) and KH is a constant (with the units

mol dm^3 atm^1 ). This equation is the mathematical expression of Henry’s law

which may be stated as follows:

The equilibrium concentration of a gas in a solvent at a particular temperature

is proportional to the partial pressure of the dissolving gas.

KHvalues are called Henry’s law constants (Table 11.6). They vary with temperature.

Most gases which chemically react with water only follow Henry’s law at very

low partial pressures (for SO 2 , less than 0.001 atm). However, since calculations in-

volving gas solubilities often involve gases (such as SO 2 and CO 2 ) which occur at

trace levels in the atmosphere, Henry’s law constants are still useful.

The solubility of a gas is unaffected by the presence of other gases unless the total

pressure of gas (i.e. dissolving gas added to other gases) is above about 1 atm

(101 kPa) pressure. Above this pressure, allgases show significant deviations from

Henry’s law because the gas mixture behaves non-ideally.

For examples of calculations involving Henry’s Law see Appendix 11 in the website.

184

Fig. 11.6The solubilities of O 2 and N 2

at different partial pressures at 25 °C.

The slopes are equal to the Henry’s law

constants for these gases.

Table 11.6Selected

Henry’s law constants

for water at 25 °C

Gas KH/mol dm^3 atm^1

O 2 1.28 10 ^3
CO 2 3.38 10 ^2
H 2 7.90 10 ^4
CH 4 1.34 10 ^3
N 2 6.48 10 ^4
NO 2.0  10 ^4

Source:Environmental
Chemistry, S.E. Manahan

Fig. 11.5According to
Henry’s Law, as the partial
pressure of the dissolvinggas
doubles, the concentration of
dissolvedgas also doubles.

Gas

dispersed

in air

Gas

dispersed

Liquid in liquid

Gas pressure