. Variation of molar conductivity with concentration Friedrich Kohlraush studied the molar conductance of different electrolytes at different concentrations. He observed that, increase of the molar conductance of an electrolytic solution with the increase in the dilution. One such experimental results is given in the following table for better understanding.
XII U9 Electro XII U9 Electro - - - - Concentration (M) Molar conductance ( Sm mol - - NaCl KCl HCl . . . .
Based on the above such results, Kohlraush deduced the following empirical relationship between the molar conductance ( Λ m and the concentration of the electrolyte (C). Λ Λ m m - k C .....( . ) The above equation represents a straight line of the form y = mx + c . Hence, the plot of Λ m Vs C gives a straight line with a negative slope of –k and the y intercept, Λ m .
Where Λ m is called the limiting molar conductivity. i.e., the molar conductance approaches a limiting value in very dilute solutions. For strong electrolytes such as KCl, NaCl etc., the plot, Λ m Vs C , gives a straight line as shown in the graph ( . ).
It is also observed that the plot is not a linear one for weak electrolytes. For a strong electrolyte, at high concentration, the number of constituent ions of the electrolyte in a given volume is high and hence the attractive force between the oppositely charged ions is also high. Moreover the ions also experience a viscous drag due to greater solvation. These factors attribute for the low molar conductivity at high concentration.
When the dilution increases, the ions are far apart and the attractive forces decrease. At infinite dilution the ions are so far apart, the interaction between them becomes insignificant and hence, the molar conductivity increases and reaches a maximum value at infinite dilution. For a weak electrolyte, at high concentration, the plot is almost parallel to concentration axis with slight increase in conductivity