Page:Outlines of Physical Chemistry - 1899.djvu/225

 Admitting this view for the present, we see that the conductivity of a solution will be proportional to the number of free ions which it contains and to the rate of migration of these ions. The expression for Kohlrausch's law will therefore not be —

H = u + v

(ji being the molecular conductivity of a salt at a given dilution, and u and v being the speeds of the ions), but—

H = x (u + v)

x being a fractional number which expresses the degree of dissociation of the salt.

As the dilution increases the value of x approaches unity, and at infinite dilution x = 1, so that

/J 00 = ^ + V

For salts containing monovalent ions the dissociation is practically complete at a dilution of 1,000 to 2,000 litres ; and a further increase in the dilution causes the conduc- tivity to become only very little higher. The same holds for certain acids and certain bases: HC1, HN0 3, KOH, NaOH, &c, and \i ^ is known for these substances.

Salts of strong acids with a weak base and of weak acids with a strong base belong to the category of easily dissociable bodies. From their investigation we can deduce the rate of migration of the ions of weak acids and of weak bases, and thus the speed of all ions can be

If we possess a complete table of values of u and v, we can find, by means of the equations

H = x (u + v)

1 If from the maximum observed conductivity of, for instance, sodium acetate, we subtract the speed of the Na ion, we obtain the speed of the C^O.; ion. — By adding to this latter the speed of the H ion we obtain h ^ for acetic acid.

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