solution that contains mol of electrolyte. In such cases L m increases steeply (Fig. . ) on dilution, especially near lower concentrations.
Therefore, L ° m cannot be obtained by extrapolation of L m to zero concentration. At infinite dilution (i.e., concentration c ® zero) electrolyte dissociates completely ( a = ), but at such low concentration the conductivity of the solution is so low that it cannot be measured accurately. Therefore, L ° m for weak electrolytes is obtained by using Kohlrausch law of independent migration of ions (Example . ).
At any concentration c , if a is the degree of dissociation Ion lllll /(S cm mol – ) Ion lllll /(S cm mol – ) H + . OH – . Na + . Cl – .
Mg + . SO . then it can be approximated to the ratio of molar conductivity L m at the concentration c to limiting molar conductivity, L m . Thus we have: ° ( .
) But we know that for a weak electrolyte like acetic acid (Class XI, Unit ), a c c c K ( . ) Applications of Kohlrausch law Using Kohlrausch law of independent migration of ions, it is possible to calculate L m for any electrolyte from the l o of individual ions. Moreover, for weak electrolytes like acetic acid it is possible to determine the value of its dissociation constant once we know the L m and L m at a given concentration c . Calculate L m for CaCl and MgSO from the data given in Table .
. We know from Kohlrausch law that CaCl + Ca Cl = . S cm mol – + ( . ) S cm mol – = ( .
+ . ) S cm mol – = . S cm mol – MgSO – + Mg SO