LAW OF MOLECULAR CONCENTRATION 129ide and its dissociation products at a temperature above 500°.
2SO 3 ^± 2SO 2 + O 2The velocity towards the right is dependent on the number of
collisions of SO 3 molecules with each other or
vel (->) = ki [BO,] [SOJ = fci [SO,]*The velocity toward the left is determined by the number of col-
lisions of three different molecules or one oxygen molecule with
two SO 2 molecules
vel («-) = kt [O 2 ] [SO,] [SO,]
= h [O 2 ] [SO 2 ]^2and the equilibrium condition is determined by the expression
[SO 2 ]^2 • [O 2 ]
[SO,]' = KInteresting as the foregoing line of reasoning is, and logical as
the deductions seem to be, the scientists who thought this out
would at once have discarded it if they had not found that it agreed
to a considerable degree of accuracy with the actual conditions
found to exist in systems in equilibrium.
Thus actual measurements of the three components in solu-
tions containing acetic acid, hydrogen ions, and acetate ions show
that, whatever the actual concentrations, the ratio always has the
same value, namely that of the equilibrium constant.
The importance of the manufacture of sulphuric acid has
caused a great amount of study to be made of the equilibrium
between sulphur dioxide, oxygen, and sulphur trioxide, and the
validity of the law of molecular concentrations, as applied to this
reaction, has been put to a rigid test.
The law is in fact of very wide application; it holds for non-
ionic as well as ionic reactions. The degree of ionization of
weakly ionized substances can be calculated with high precision ac-
cording to the law. But the behavior of strong electrolytes does
not conform as closely to this law, and the law is of value only
in a qualitative fashion to predict the extent of the ionization
of these substances. In this connection we may recall Rule 4
for writing ionized equations, which directed to treat all strong
electrolytes as if they were completely ionized.