200 OUTLINES OF PHYSICAL CHEMISTRY
But as we are working with a solution of known specific conductivity I, we have the relation
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��l = k x
��n.db'
��from which k can be calculated, and that value is called the capacity of the electrolytic cell employed. This capacity is the factor by which all the direct observations made with this cell must be multiplied so as to bring the specific conductivities into accord with our definition.
If now the same cell be filled, without altering the distance between the electrodes, with a solution of unknown conductivity, and the point d' found at which the telephone is silent, then we get the new value
I = aa " s' b'. d' V
and multiplying by k we find the specific conductivity wanted. The specific conductivity multiplied by v (the number of cubic centimetres in which one gram-molecule of substance is dissolved) gives the molecular conductivity :
__ , v .ad'
Bemarks.—l. In practice, it is common to refer the conductivity, not to the molecular weight, but to the equivalent of the dissolved substance.
2. The conductivity of electrolytes generally increases with the temperature (probably on account of the increased mobility of the ions). However, a series of substances is known which behave in the opposite manner. Certain acids, such as sulphuric, orthophosphoric, and hydrofluoric acids, belong to this series, and they are all characterised
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1 We admit that the hydrolytic dissociation of these acids takes place exothermically. Now, according to the principle laid down on p. 181, an elevation of temperature must give rise to an endothermio
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