Page 178 - Chemical equilibria Volume 4

154 Chemical Equilibria
Yet if the solution is sufficiently dilute, and if v denotes the molar
volume of the pure solvent, we can write: 0 0
∑ x V << x V ≅ V ≅ v 0 0 [A1.4]
s
s
0
0
0
s
Thus, the molarity can be written approximately as:
x
C ≅ s [A1.5]
s
v 0
0
The molar volume depends on the temperature and pressure for a gas. It
depends practically only on the temperature for a liquid or a solid.

A1.2. Chemical potentials and activity coefficients

We know that the chemical potential of a component in a perfect solution
is written in the form:
μ i = μ i 0 + Rln x i [A1.6]
T

For a real solution, Lewis refers to that expression of the chemical
potential, attempting to preserve its form. In order to do this, he introduced
an activity coefficient γ, as a function of the temperature, pressure, etc. and
of the solution’s composition, writing the chemical potential of a component
of the solution in the form:

μ i = μ i 0 + Rln x [A1.7]
γ
T
i i
The product of the activity coefficient and the molar fraction is known as
the activity of the component in the solution. The activity in a real solution
plays the same thermodynamic role as does the molar fraction in a perfect
solution:
a = γ i i [A1.8]
x
i