254 OUTLINES OF PHYSICAL CHEMISTRY

out by any of the methods by which the degree of dissocia- tion is ascertained. The simplest way is by determining the electric conductivity.

In order to cite an example, suppose we have a solution containing one gram-equivalent of acetic acid in v litres. We know the maximum molecular conductivity (the value fi w , page 201) of this acid, and the molecular conductivity of the solution in question can be experimentally deter- mined. The ratio -^- is equal to the degree of dissociation

and tells us that a fraction x of the dissolved equivalent has suffered dissociation whilst the remainder 1 — x of the equivalent has not been changed. The concentration u x of the ionised molecules, that is, their number of equivalents

per litre of solution, is, therefore, expressed by — , and the

1— x

concentration u of the undissociated acid will be ■.

v

By introducing these values into the equation of equili- brium, we get

��k x (1 — x)y

We notice that the value of k is a function of the volume v, and this offers us a means of checking the exactness of the theory.

For acetic acid at 25°C, /i = 364 — deduced from the molecular conductivity of sodium acetate.

By determining the molecular conductivity of free acetic acid for a series of solutions containing one gram-equivalent

in v, v l ,v 2 , litres, we can calculate the ratio _ ^- in

each case, and then we have a series of degrees of dissocia- tion x, x u x 2 , We might then examine if the correla- tive values of vand ofx satisfy the equation of equilibrium. Ostwald has made this study, and his results, given in the following table, show admirably the constancy of k :

�� �