Example 2.5

Example . Both conductivity and molar conductivity change with the concentration of the electrolyte. Conductivity always decreases with decrease in concentration both, for weak and strong electrolytes. This can be explained by the fact that the number of ions per unit volume that carry the current in a solution decreases on dilution. The conductivity of a solution at any given concentration is the conductance of one unit volume of solution kept between two . . Variation of Conductivity and Molar Conductivity with Concentration platinum electrodes with unit area of cross section and at a distance of unit length. This is clear from the equation: A G   l (both A and l are unity in their appropriate units in m or cm) Molar conductivity of a solution at a given concentration is the conductance of the volume V of solution containing one mole of electrolyte kept between two electrodes with area of cross section A and distance of unit length. Therefore, κ Λ κ A l Since l = and A = V ( volume containing mole of electrolyte) L m = k V ( . ) Molar conductivity increases with decrease in concentration. This is because the total volume, V , of solution containing one mole of electrolyte also increases. It has been found that decrease in k on dilution of a solution is more than compensated by increase in its volume. Physically, it means that at a given concentration, L m can be defined as the conductance of the electrolytic solution kept between the electrodes of a conductivity cell at unit distance but having area of cross section large enough to accommodate sufficient volume of solution that contains one mole of the electrolyte. When concentration approaches zero, the molar conductivity is known as limiting molar conductivity and is represented by the symbol L ° m . The variation in L m with concentration is different (Fig. . ) for strong and weak electrolytes. Strong Electrolytes For strong electrolytes, L m increases slowly with dilution and can be represented by the equation: L m = L ° m –