1 62 DEGREE OF DISSOCIATION. chap.
and Raoiilt. For higher concentmtions is always greater
than the calculated value; the following salts, however, which are known to partially form double molecules in concentrated solution, behave exceptionally in this matter: cadmium iodide, ihagnesium sulphate, zinc sulphate, copper sulphate, etc. (compare p. 143).
The more recent determinations by Hausrath (6*) show that the degrees of dissociation at high dilution obtained from the freezing point experiments agree well with those found from the conductivities.
Dissociation Equilibrium of Weak Electrolytes. — The laws mentioned in Chapter YL for ordinary dissociation must also obtain for the equilibrium between an electrolyte and its ions. As already shown, the dissociation of a substance AB which decomposes into the components A and B is regulated by the law of mass action —
K X C\^ = C, X On
where ^ is a constant.
If we dissolve an electrolyte, for instance, acetic acid,
+ —
it partially dissociates into the ions H and CHsCOi, and the
above law can be applied to this decomposition. This was
done by Ostwald, and almost simultaneously by van't Hoflf
(7), whose results for acetic acid at 14' 1° are given in the
'next table, ft denotes the molecular conductivity, and a
(observed) the degree of dissociation calculated from this.
Under a (calculated) is given the degree of dissociation
calculated by means of the above formula, setting C\ = f b,
since both ions must occur in equal quantities; v is the
dilution, i.e. the volume in litres in which a gnim-molecule
(GO grams) of acetic acid is dissolved.
Since —
- /' ^ - " onJ r - /' - "
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