Limiting Molar Conductances of Weak Electrolytes

Limiting Molar Conductances of Weak Electrolytes

One of the immediate application of Kohlrausch’s law at infinite dilution is the  determination of the limiting molar conductance at infinite dilution _m  of weak electrolytes. These values cannot be determined by extrapolation of a plot of _m versus √C (see fig 2) as in the case of strong electrolytes. In the case of weak electrolytes, an indirect method is employed

For example in the case of acetic acid, its _m  may be determined indirectly as follows;

_m ( CH₃COOH) = _m  (CH₃COONa) + = _m (HCl) - _m (NaCl)

=91.0+425.0-126.5

= 389.5^-1cm²mol^-1

or simply;

_m ( CH₃COOH) =  _CH3COO- + _H⁺

= 40.9 + 348.6

= 389.5 ^-1cm²mol^-1