Page 181 - Modern physical chemistry

8.2 Useful Equilibria Expressions 17l

When concentrations are expressed in molalities, we define

[8.19J

and employ
[8.20J
Then we have
[8.21 J
with
I m
Q _ YL YM [8.22J
r - a b·
YA YB
The different equilibrium constants for a reaction are related in the following manner.
For a very dilute solution, equations (6.65) reduce to

[8.23J

or
1000 Xi =mi = Ci. [8.24J
Ms d
So when A, B, L, and M are all solutes, we have

[8.25J

where
1m =l+m-a-b. [8.26J
Introducing (8.11), (8.15), and (8.19) leads to

(1~~ rn Kx =Km =(~rn Kc. [8.27J

From the last equation and the equalities

[8.28J
which hold in infinitely dilute solutions, we obtain

Kpn =[d~rn K~. [8.29J

Here d s is the mass density of the solvent. The difference between these two constants
is small in aqueous solutions, where d s is nearly 1.
By convention, the activity of a solvent is generally expressed in the mole fraction system.
So in dilute solutions, its activity is set equal to 1. For the S<Kalled ionization of water
[8.30J