CHAPTER 9 MIXTURES
9.7 ACTIVITY OF ANUNCHARGEDSPECIES 272
Table 9.5 Expressions for activities of nonelectrolytes. For a con-
stituent of a condensed-phase mixture,fi,fA, andfBrefer to the
fugacity in a gas phase equilibrated with the condensed phase.
Substance Activity
Pure gas a(g)D (g)D
f
p
Pure liquid or solid aD
Substanceiin a gas mix-
ture
ai(g)D i(g)i
pi
p
D
fi
p
Substanceiin a liquid or
solid mixture
aiD i (^) ixiD i
fi
fi
Solvent A of a solution aAD A (^) AxAD A
fA
fA
Solute B, mole fraction
basis
ax;BD x;B (^) x;BxBD x;B
fB
kH,B
Solute B, concentration
basis
ac;BD c;B (^) c;B
cB
c
D c;B
fB
kc;Bc
Solute B, molality basis am;BD m;B (^) m;B
mB
m
D m;B
fB
km;Bm
9.7.3 Pressure factors and pressure
At a given temperature, the pressure factor iof componentiof a mixture is a function
only of pressure. To derive the pressure dependence of ifor various kinds of mixture
components, we need expressions for.refi i/as functions of pressure to substitute in
the defining equation iDexpå .refi i/=RT ç.
For componentiof agas mixture, the reference state is pure gasiat the pressure of
the mixture, behaving as an ideal gas. The chemical potential of a pure ideal gas depends
on its pressure according to Eq.7.8.6:D(g)CRTln.p=p/. Thus the chemical
potential of the reference state of gas componentiisrefi (g)Di(g)CRTln.p=p/,
andrefi (g) i(g) is equal toRTln.p=p/. This gives us the following expression for the
pressure dependence of the pressure factor: