The ionization constant can be used to calculate the degree of dissociation or ionization and the concentration of H+ ions.
Degree of dissociation or ionization may be defined as:
The fraction of the total number of molecules of an electrolyte (acid or base) which ionizes into ions
It is generally denoted by the symbol α (alpha).
Thus,
Degree of ionization, α (alpha) = [Number of molecules of the electrolytes which ionize ] / [Total number of molecules of the electrolytes]
If c is the number of moles of the acid in one litre of the solution and α is the degree of ionization, then the concentrations of each species at equilibrium are:
HA + H2 < ----------> H3O+ + A–
Equilibrium concentration:
[HA] = c (1 – α)
[H3O+] = c α
[A–] = c α
Therefore, Ka = H3O+] [A–] / [HA]
= (c α) X (c α) / c (1 – α)
But for weak acids, α is very small so that 1 – α ≈ 1
Therefore,
Ka = (c α) 2
α= (Ka / c) 1/2
And
H3O+ = c x (Ka / c) 1/2
H3O+ = (Ka. c) 1/2
