268 OUTLINES OF PHYSICAL CHEMISTRY
��of each of the ionised molecules C 2 H 3 2 H and HOH.
The hydrochloric acid must be considered as totally
— +
dissociated into HOI and HOH. In order that the
+ common molecules HOH should have the same concentra- tion in both solutions, one equivalent of hydrochloric acid must be dissolved in such a volume of water that
1 = 0-0015, that is, in 667 litres. v
Avidity and Degree op Dissociation
It has already been pointed out that the avidity of acids is proportional to their degree of dissociation, and seems to depend only on the ionised molecules. In other words, the degree of activity with which an acid ionises molecules of water is the same as that which it exerts when brought into contact with molecules of a basic nature. In the case of the action of two acids on one base, the acid with the greater ionising power will also have the greater action on the base.
But we may try to study this question in a more thorough manner. In a solution containing one equi- valent of hydrochloric acid and one equivalent of a weak acid HA (acetic acid, for example), these two electrolytes are unequally dissociated : the system 1 equivalent \ n ^^ on .
_ + L (1 + x) equivalent
/ ha<-~*hoh)
1 equivalent i
- HA
contains few unitary molecules HC1 and many HA
molecules.
Suppose now that one equivalent of caustic soda takes
1 By the reaction
(HOH + RH) + (HOM + HOH) = (HR + MOH) + 2HOH.
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