The conductivity of an electrolytic solution changes with the change in the concentration of an electrolyte as the number of ions in one cm cube of the solution changes with the variation in concentration. To compare the conductivity of different electrolytes, it is essential that the solutions should be of equivalent concentrations i.e. capable of furnishing ions carrying the same total charge of electricity. For example- one gm equivalent of different electrolytes in the solution produces ions having the same total charge of electricity. Thus, the conductivities of different electrolytes are compared by expressing them as Equivalent Conductivity which is defined as “the conductance of a solution containing 1 gm equivalent of electrolyte at a specified concentration (or dilution) and temperature such that the whole of the solution is placed between two parallel electrodes one can apart”. It is denoted by Λeq.
Relation between Conductivity and Equivalent Conductivity:
Consider V cm3 of the solution containing 1 gm equivalent of the electrolyte placed between two electrodes 1 cm apart.
Let the measured conductance of the solution = x = Equivalent Conductance Λeq [because the solution contains 1 gm equivalent of electrolyte].
Number of cm cubes in V cm3 = V
Now, conductivity is the conductance of 1 cm cube of solution.
Therefore, κ = x/V = Λeq/V
or Λeq = κV, where ‘V’ is the volume containing 1 gm equivalent of the electrolyte.
If ‘C’ is the concentration of the solution in gm equivalent per litre i.e. normality of the solution, then C gm equivalents of electrolyte are present in = 1000 cm3 of solution
1 gm equivalent of electrolyte is present in = 1000/C cm3 of solution
Therefore, Λeq = κ x 1000 / C = κ x 1000 / N, where ‘N’ is the normality.
|Units: ohm-1 cm-1 x cm3 / gm equivalent|
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