CHAPTER 9 MIXTURES
9.6 EVALUATION OFACTIVITYCOEFFICIENTS 263
b
bc
bc fA
xAfA
0 0:2 0:4 0:6 0:8 1:0
0
2
4
6
8
xA
fA
=kPa
(a)
bc
0 0:2 0:4 0:6 0:8 1:0
0
1
2
3
4
xA
A
(b)
Figure 9.9 Liquid mixtures of ethanol (A) and H 2 O at 25 C and 1 bar.
(a) Ethanol fugacity as a function of mixture composition. The dashed line is Raoult’s
law behavior, and the filled circle is the pure-liquid reference state.
(b) Ethanol activity coefficient as a function of mixture composition.
Table 9.4 Activity coefficients as functions of fugac-
ity. For a constituent of a condensed-phase mixture,fi,
fA, andfBrefer to the fugacity in a gas phase equili-
brated with the condensed phase.
Substance Activity coefficient
Substanceiin a gas mixture iD
fi
pi
Substanceiin a liquid or solid
mixture
(^) iD
fi
xifi
Solvent A of a solution (^) AD
fA
xAfA
Solute B, mole fraction basis (^) x;BD
fB
kH,BxB
Solute B, concentration basis (^) c;BD
fB
kc;BcB
Solute B, molality basis (^) m;BD
fB
km;BmB
Figure9.9(b) shows how (^) Avaries with composition. The open circle is atxAD0:4
and (^) ADfA=.xAfA/. Note how (^) Aapproaches 1 asxAapproaches 1 , as it must according
to Eq.9.5.20.
Water and 1-butanol are two liquids that do not mix in all proportions; that is, 1-butanol
has limited solubility in water. Figures9.10(a) and9.10(b) on the next page show the
fugacity of 1-butanol plotted as functions of both mole fraction and molality. The figures
demonstrate how, treating 1-butanol as a solute, we locate the solute reference state by a