Example 2.6

Example . Fig. . : Variation of L m against c½. Kohlrausch examined L ° m values for a number of strong electrolytes and observed certain regularities. He noted that the difference in L ° m of the electrolytes NaX and KX for any X is nearly constant. For example at K: L ° m (KCl) – L ° m (NaCl) = L ° m (KBr) – L ° m (NaBr) = L ° m (KI) – L ° m (NaI) ≃ . S cm mol – and similarly it was found that L ° m (NaBr) – L ° m (NaCl) = L ° m (KBr) – L ° m (KCl) ≃ . S cm mol – On the basis of the above observations he enunciated Kohlrausch law of independent migration of ions . The law states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte . Thus, if l ° Na + and l ° Cl – are limiting molar conductivity of the sodium and chloride ions respectively, then the limiting molar conductivity for sodium chloride is given by the equation: L ° m (NaCl) = l ° Na + + l ° Cl ( . ) In general, if an electrolyte on dissociation gives n + cations and n – anions then its limiting molar conductivity is given by: L ° m = n + l ° + + n – l ° ( . ) Here, l ° + and l ° – are the limiting molar conductivities of the cation and anion respectively. The values of l ° for some cations and anions at K are given in Table . . Table . : Limiting Molar Conductivity for some Ions in Water at K Weak Electrolytes Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in L m with dilution is due to increase in the degree of dissociation and consequently the number of ions in total volume of