Kohlrausch’s Law | Application




The law states that at infinite dilution, where ionization of all electrolytes is complete and where all interionic effects are absent, each ion migrates independently of its co-ion, and contributes a definite share to the total equivalent conductance of the electrolyte. In other words, Λo for any electrolyte should be the sum of the contributions of the equivalent conductance of the individual ions. If the equivalent conductance of a cation at infinite dilution is λo+ and that for the anion it is λo then according to the above law:




Kohlrauschs law suggests that at the infinite dilution the conductance of an electrolyte, KCl for example, depends on independent contributions from and ions. The independence of these contributions is seen from the difference between values of electrolytes containing a common ion (Table 12.3). For example, when salts of potassium and lithium with a common anion are taken, the difference between λoK+ and λo Li + is constant. The same is also true for salt with a common cation i.e.,


The table shows that these differences are independent of other ionic species present. This law gives no way of finding the contributions of individual ions.

Application of Kohlrausch’s Law.

1.Calculations of Λo for weak electrolytes
It has already been pointed out that direct determination of the equivalent conductance of a weak electrolyte at infinite dilution is not possible. In fact, the law of independent migration of ions is used for calculating the limiting equivalent conductance (Λo) for weak electrolytes. Thus in calculating for acetic acid, the equivalent conductance of three strong electrolytes, namely, HCl, NaCl, and NaAc should be known. Therefore one can write:



Since the quantities on the right-hand side of equation 12.20 are experimentally determined and are known, hence can be calculated.
Example 12.5. At 298 k, the values of ionic conductance at infinite dilution of HCl, NaAc, and NaCl are 0.042616, 0.01265 ohm-1 m2  respectively. Calculated the equivalent conductance of acetic acid at infinite dilution.
Calculations of Λo for weak electrolytes


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