Variation of molar conductance with dilution

Variation of molar conductance with dilution

In 1900 Kohlrausch derived an empirical equation which relates the molar conductance _m at concentration C to the limiting molar conductance at infinite dilution _m :

_m = _m  -                                                                                                             (9)

K is a constant which depends on the nature of the salt (e.g. whether it is of the form MA, M₂A, etc.) than its specific identity.

Another equation which relates molar conductance at a certain concentration to that at infinite dilution was also derived by Debye and Huckel but was later modified by Onsager and hence became to be known as the Debye-Huckel-Onsager equation or simply the Onsager equation:

_m = _m  -                                                                         (10)

where A and B are constants which depend on temperature and the nature of the solvent. The Onsager equation holds true for solutions of concentrations up to 10^-1M.

The molar conductances of strong electrolytes eg: KC1 depends weakly on concentration. This class of electrolytes exhibit fairly high values of molar conductance over the whole concentration range except at very low concentrations where there is a slight increase in conductance. At infinite dilution the molar conductance reaches the limiting value, _m .

In the case of weak electrolytes e.g. CH₃COOH, there is strong dependence of molar conductance on concentration. At high concentrations they exhibit low values of _m but at very low concentrations the value of _m increases steadily to high values comparable to those of strong electrolytes. At infinite dilution the molar conductance reaches the limiting value, _m .

In between strong and weak electrolytes, lies the class of intermediate electrolytes whose values of molar conductances are in between those of strong and weak electrolytes. These are mostly salts of transition metals, e g ZnSO₄ (Fig  2)