EXTENT OF IONIZATION 99The degree of ionization of a salt may be calculated from the
freezing point of its solution. For example, if 0.1 mole of K2SO4
(17.4 grams) dissolved in 1,000 grams of water freezes at —0.454°,
we know that 0.1 mole of un-ionized substance will lower the
freezing point 0.1 X 1.86 = 0.186°, and that 0.1 mole of a com-
pletely ionized substance giving 3 ions will lower the freezing
point 3 X 0.186 = 0.558°. The actual freezing-point lowering,
0.454°, lies between these values and thus indicates incomplete
ionization. The proportion of the salt ionized is found by di-
viding the difference caused by the actual ionization, 0.454 — 0.186
= 0.268°, by that which would be caused by 100 per cent ion-
ization, 0.558 - 0.186 = 0.372°, thus giving 0.268/0.372 = 0.72.
Thus the salt is 72 per cent ionized.
- If 9.45 grams of chloracetic acid, H(C 2 H 2 O 2 C1), dis-
solved in 1,000 grams of water show an osmotic pressure of
2.51 atmospheres, find the per cent ionization of the acid,
assuming that only one hydrogen is ionizable and that the
negative ion has the composition of the radical shown in the
parenthesis. - A solution of 101 grams of potassium nitrate in 1,000
grams of water freezes at —3.05. Calculate what percentage
of the salt is ionized.
EXTENT OF IONIZATION
In the last section the method was illustrated by which per
cent ionization could be calculated from freezing point on the
assumption that ions and un-ionized molecules are of equal effect.
In the following tables are given the values calculated for the
ionization of various electrolytes in 0.1 equivalent solution on
the basis of this assumption. These values we shall designate as
the apparent ionizations. They come out approximately the same
whether they are calculated from electrical-conductivity measure-
ments or from freezing points.
Since the solvent is undeniably the agent that facilitates ioniza-
tion, it is obvious that the more solvent, or in other words the more
dilute the solution, the greater the percentage of ionization. In
very dilute solutions (1 mole of solute in 10,000 liters of solvent)
strong electrolytes are practically completely ionized. In solu-
tions more than 0.1 equivalent the apparent ionization is less than