LAW OF MOLECULAR CONCENTRATION 127temperatures. The word molecule here signifies any individual
component whether electrically charged or not. Thus, a solution
of acetic acid would contain the components H+, C2H3O2, and
HC2H3O2. Concentration signifies the amount of the component
divided by the volume and is usually expressed in gram molecu-
lar weights per liter. For example, take 100 cc. of a solution
containing 0.6 gram (0.01 mole) of HC2H3O2. Dividing the
amount (0.01 mole) by the volume, 0.1 liter, gives the ratio 0.1,
and thus the concentration of the total acetic acid is 0.1 F.W.
(formula weight) per liter. Since the fraction ionized is 0.014 the
concentration of hydrogen ion, [H+],* is 0.0014; the concentration
of the acetate ions [C2H 3 O 2 ~] is also 0.0014, and the concentration
of un-ionized acetic acid [HC2H3O2] is 0.1 - 0.0014 = 0.0986.
Derivation of the Law of Molecular Concentration from a Con-
sideration of the Speed of Reaction. Let us consider again the
general reaction
A+B^C+D
It is obvious that for a molecule of A to react with a molecule of
B the two must come into contact or collide. The chance for
collision of any single molecule of A with molecules of B is pro-
portional to the number of B in a given volume, that is, to the
concentration of B, but there are a great many molecules of A
each of which has the same chance to collide with molecules of B.
Therefore, the total number of collisions is proportional to the
product of the concentrations of A and B. However, the velocity of
the reaction towards the right, that is, the amount changed in
unit time in unit volume, is proportional to the number of collisions.
Thus
Velocity (-») = jfe, [A] [B]
The factor fci is a definite numerical value known as the propor-
tionality constant.
As the reaction progresses and C and D accumulate it is ob-
vious that collisions between C and D will ensue, and an exactly
similar consideration will show that the velocity in the opposite
direction is given by the expression
Velocity (*-) = k 2 [C] [D]
- In mathematical equations the formula enclosed in brackets signifies
the concentration of the substance.