128 CONDUCTIVITY OF ELECTROLYTES. chap.
units. The specific resistance changes with the concentration, temperature, and pressure. The specific electrical conductivity is the reciprocal of the specific resistance. Expressed in the new units it is 10630 times greater than when the older units are used. The specific conductivity divided by the concentration gives the molecular condttctiviti/. As unit of concentration, use is frequently made of the number of molecules in 1 c.c. instead of in 1 litre. The molecular conductivity expressed in this way is 1*063 x 10'^ times greater than when expressed in the old units (S.U., column 1 metre long and of cross section 1 sq. mm., mols per litre). We shall here make use of the new units.
If the degree of dissociation of the solution did not change with dilution, i.e, if the percentage of molecules dissociated into ions were independent of the dilution, the molecular conductivity (the specific conductivity per gram-molecule) would also be constant. The changes in the value of the molecular conductivity give, therefore — for not too high concentrations — a measure of the increase in the dissociation by dilution.^
The specific electrical conductivity is denoted by k. As this, however, changes with the dilution (i.e. with the volume V in litres in which a gram-molecule is dissolved), it is customary to add to k an index denoting this volume, and K, then expresses the specific conductivity at dilution v. For the equivalent conductivity the symbol X is used, and this also is provided with an index v indicating the dilution. According to the above definitions there exists the following connection between k, X, and ij (number of equivalents per c.c.):
Xp also changes with the dilution, and at infinite dilution
^ When the coDcentrations are high (so that the number of dissoh'cd molecules cannot be neglected) this alteration of the molecular conduc- tivity cannot be used as a measure of the change of degree of dissociation. For ^-normal and more dilute solutions, however, it is generally valid.
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