Page:Outlines of Physical Chemistry - 1899.djvu/277

 The part played by the base towards one or the other acid is a matter of indifference, because the sodium salts must be considered as completely dissociated. We shall then have :

1 equivalent i HC1< — >HOH

(hoi +

% NaOH 1 equivalent, 'HA 1 '

HA <— > HOH

��1 equivalent

��HA

��and in this system the equilibrium depends on the quantities of the two acids remaining in the state of simple aqueous solution. In order that these remaining parts may not in any way change the equilibrium esta- blished, they must have the same activity or force ; that is to say, they must react by means of the same number of ionised molecules and be isohydric.

If HA represents acetic acid and if the total volume of the solution is 7 to 8 litres, the remaining parts of the two acids must be in the proportion of about 0*0015 (HC1)

To account for these facts Thomsen says : When there is competition between one equivalent of hydrochloric acid and one equivalent of acetic acid for one equivalent of caustic soda, the acids unite with quantities of the base which are in the proportion of 0*9985 (or 1) to 0*0015. It would be better to say : the remaining quantities of the acids, that is, the parts which continue to behave simply as substances dissolved in water, are in the proportion of 0*0015 to 0*9985. These two statements may appear to be identical ; they are, however, very different. Only the latter is in conformity with our theories, and it alone

��1 See the calculation on p. 257.

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