Concise Physical Chemistry

c13 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

MOLAR CONDUCTIVITY 207

HCl

HOAc

NaOAc

c1/2

FIGURE 13.3 Kohlrausch’s law for conductance of the strong electrolytes HCl and NaOAc

and the weak electrolyte HOAc.◦is the intercept on the vertical axis.

electrostatic interference from crowding by other ions that may be of the same elec-

trolyte or any other electrolytes present in the solution. For example, the Kohlrausch

curves for the NaOAc and HCl in Fig. 13.3 can be extrapolated to a linear approxi-

mation shown by the dashed lines. The intercept of this limiting straight line with the

vertical axis is◦.Itiscalledthemolar conductivity at infinite dilution. The shape

of the HOAc curve, however, is such that one would not wish to find◦in the same

way for this solute as for HCl and NaOAc because the curve is too steep near

√

c= 0

to do the job with acceptable accuracy.

Electrolytes in aqueous solution generally fall into two broad classes:strong

electrolytes, which show clean linear extrapolations to◦, andweak electrolytes,

which do not. A collection of experimental values for◦for strong electrolytes runs

from about 100 to 425 ohm−^1 and, other than high values for strong acids and bases,

shows no particular regularity in itself. Kohlrausch observed, however, that many

differencesin◦are equal; for example,

◦KCl−◦NaCl=◦KNO 3 −◦NaNO 3 =◦KOH−◦NaOH

and so on, for a number of other◦values arranged pairwise in this fashion. This

would be true, he said, if◦values were the result of contributions from each of the

ions produced when the strong electrolyte goes into solution; for example,

NaCl(s)→Na+(aq)+Cl−(aq)

where (s) designates the solid compound and (aq) designates the ion in aqueous

solution. If this were true, he said, the regularity in◦could be explained by writing