For a general reaction, in which a moles of substance A and
b moles of B react to form c moles of C and d
moles of D,
a A + b B
c C + d D
the equilibrium-constant expression is
This is the law of mass action.
Concentrations can be measured in any terms that express the relative
numbers of molecules per unit volume. In solutions, concentrations
usually are given in molarity, or moles per liter, .
This is indicated by a subscript C to the equilibrium constant:
Gases sometimes are measured in moles per liter also, but more often
are measured in mole fractions, ,
or in partial pressures, .
(The subscript j simply refers to any species.) The equilibrium
constants using these units are indicated by
The mole fraction of the
jth component of a gas mixture, ,
is the number of moles of that particular gas, divided by the total
number of moles of all gases present:
Our atmosphere has one molecule of
for every four molecules of
; thus the mole fraction of oxygen gas is
and that of nitrogen is
The sum of mole fractions of all the gases in a mixture, of course,
must be exactly one.