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PHASE EQUILIBRIA INVOLVING VAPOUR PRESSURE 223

Table 5.4 Henry’s law constants
◦
k H for gases in water at 25 C
Gas k H /mol dm −3 bar −1
CO 2 3.38 × 10 −2
O 2 1.28 × 10 −3
CH 4 1.34 × 10 −3
N 2 6.48 × 10 −4

partial pressure) of the gas above it. Stated in another form, Henry’s
law says: One of the simplest
ways of removing
(5.20) gaseous oxygen from
[i (soln) ] = k H p i
water is to bubble
where p i is the partial pressure of the gas i, and [i (soln) ]isthe nitrogen gas through
concentration of the material i in solution. The constant of propor- it (a process called
tionality k H is the respective value of Henry’s constant for the gas, ‘sparging’).
which relates to the solubility of the gas in the medium of choice.
Table 5.4 lists a few Henry’s law constants, which relate to the
solubility of gases in water.
Strictly, Henry’s law
Worked Example 5.7 What is the concentration of molecular oxy- only holds for dilute
◦
gen in water at 25 C? The atmosphere above the water has a pressure systems, typically in
5
of 10 Pa and contains 21 per cent of oxygen. the mole-fraction range
0–2 per cent. The law
Strategy. (1) We calculate the partial pressure of oxygen p (O 2 ) .(2) We tends to break down
calculate the concentration [O 2(aq) ] using Henry’s law, Equation as the mole fraction
(5.20), [O 2(aq) ] = p (O 2 ) × k H(O 2 ) . xincreases.

(1) From the partial of oxygen p (O 2 ) = x (O 2 ) × the total pressure p (total) ,where x is
the mole fraction:
5
p (O 2 ) = 0.21 × 10 Pa
4
p (O 2 ) = 2.1 × 10 Pa or 0.21 bar
(2) To obtain the concentration of oxygen, we insert values into Henry’s law,
Equation (5.20):
O
[O 2(aq) ] = 0.21 × p × 1.28 × 10 −3 mol dm −3 bar −1
[O 2(aq) ] = 2.69 × 10 −4 mol dm −3
This relatively high
concentration of oxy-
We need to be aware that k H is an equilibrium constant, so
◦
its value depends strongly on temperature. For example, at 35 C, gen helps explain why
fish can survive in
water only accommodates 7.03 mg of oxygen per litre, which ex-
water.
plains why fish in warm water sometimes die from oxygen
starvation.