CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 206
- At low pressures, the gaseous phase may be approximated by an ideal gas, and the
fugacity of the component may be replaced by its partial pressure. Equation (7.40) thus
becomes
p 2 =p y 2 =KHx 2 , (7.42)
wherey 2 is the molar fraction of component 2 in the gas. Equation (7.42) is sometimes
also called Henry’s law.
- If the liquid phase is an ideal solution, the gaseous phase is an ideal gas, andx 1 →1, the
following relation between the partial pressure of component 2 and the total pressure
applies
p 2 =p y 2 =p−ps 1 , (7.43)
whereps 1 is the saturated vapour pressure of component 1.
Example
0.05 mol CO 2 dissolves in 100 g of water (n = 5.551 mol) at 4◦C and the normal pressure.
Calculate Henry’s constant ifps 1 (4◦C) = 813Pa.
Solution
The molar fraction of CO 2 in liquid is
xCO 2 =
0. 05
5 .551 + 0. 05
= 0. 00893.
From relations (7.42) and (7.43) we obtain
KH=
pCO 2
xCO 2
=
p−psH 2 O
xCO 2
=
101325 − 813
0. 00893
= 11. 26 MPa.
Henry’s law is approximate because it assumes independence ofKHon composition. It is based
on the assumption thatx 2 1 , i.e. that the solubility of gas in liquid is low.
