Page:Text-book of Electrochemistry.djvu/152

 dilution the dissociation is nearly complete, or the degree of dissociation, i.e. the proportion of molecules dissociated into ions, approaches the value 1.

From this we may conclude that the conductivity at infinite dilution, that is, when v becomes excessively large, has a value which is not very different from that for the highest measured dilution ; it can be found by graphical extrapolation, and is denoted by X„. The extrapolation can also be calculated, e.g. for KCl, with the aid of the following differences : —

Xioo - Aio = 10-4.

Xiooo — Xioo = 4*91.

Xioooo — Aiooo = 1'73.

These differences decrease almost in geometrical pro- gression.

The value of A„ for all highly dissociated electrolytes can be determined in the same way.

According to principles derived below from Kohlrausch's law, the values of the differences for various electrolytes are approximately the same. In the table on p. 135 the value of X„ for some salts is given.

Calculation of the Degree of Dissociation in Electro- lytic Solutions. — From what has been said, it is easy to see how the degree of dissociation of an electrolyte at any particular dilution 'i? is to be calculated. If all the dissolved molecules took part in the conduction of the current, X, for each single salt would be independent of the dilution, and in the case of potassium chloride it would be equal to 13011.

Since all the K ions transport the electricity at the same rate, and the same is true for the CI ions, then if the value of X is different firom that of X„, the transportation of the

electricity must be carried out by ^ - ions, i.e. the degree of

dissociation a is given by —

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