(i) Ohm’s Law: It states “the current (I) flowing through a conductor at
a given temperature, is proportional to the potential difference (V) and inversely
proportional to the resistance (R)”. Both the metallic as well as electrolytic conductors
obey ohm’s law.
(ii) Resistances (R) : It is the property of a substance by which it obstructs
the flow of current through it. Its SI unit is ohm
is fixed for a conductor, called cell constant and
(Rho), the constant of proportionality is called the Resistivity
or specific resistance.
Hence, Resistance = Resistivity × cell constant.
Thus, Resistivity of a uniform conductor is equal to its resistance when its length
is 1 m and its cross-sectional area is
(iii) Conductance (C): It is the reciprocal of the electrical resistance,
. It is expressed as
or mho and its unit is siemen,
Conductivity or specific conductance
is the reciprocal of resistivity or specific resistance
. It is the conductance between opposite faces of 1 m cube
Thus, conductivity of a uniform conductor is equal to its conductance when its length
is 1 m and cross-sectional area is
(iv) Molar conductivity
: The conductivity produced by dissolving 1 gram-mole
of an electrolyte placed between two large electrodes at one centimeter apart.
Where, V = Volume of solutions
containing one mole of electrolyte
(v) Equivalent conductivity
: The conductivity produced by dissolving one gram
equivalent of an electrolyte in solution placed between two large electrodes one
Where, V = Volume of solution
containing 1 g equivalent of the electrolyte
(vi) Factors affecting molar as well as equivalent conductivity :
(A) Nature of electrolyte i.e. strong or weak.
(C) Concentration of electrolytes in solution.
(vii) Relation between Molar conductivity
and Equivalent conductivity
(viii) Variation of Molar Conductivity with Concentration
For strong electrolyte like KCl, HCl etc. the variation is given by the following
From the above plot it is clear that
(a) Molar conductivity of a strong electrolyte (KCl) increases to a small extent
with dilution due to decrease in interionic attraction.
(b) Molar conductivity of a weak electrolyte
also increase with dilution, but the magnitude of increase of
for weak electrolyte is much larger than that of a strong electrolyte.
From the above plot, it is possible to determine
of a strong electrolyte by extrapolation of graph of
which is not possible in case of a weak electrolyte.
Conductivity ratio : it is the ratio of
at any dilution to that of infinite dilution
(ix) Effect of Temperature and Pressure on Conductivity :
The conductivity of all electrolytes increases with temperature. However, the conductivity
slightly varies with the pressure due to change in viscosity of the medium. This
alters the speed and hence the conductivity of ions.
The viscosity of dilute solution decreases with increasing the pressure. As a result
increases provided the pressure is not too high.
Key Concept :
We know that
as well as
of a solution of both weak/strong electrolytes increases with dilution. But, specific
conductivity decreases with dilution due to the decrease in number of current carrying
. At infinite dilution, a limiting value of conductivity
for any strong electrolyte is calculated by graphical method while the same for
weak electrolyte is determined by Kohlarausch’s law.
Example 1. A column of diameter 1 cm and length 50 cm is filled with 0.05
solutions. The resistance of the column is found to be
Calculate its resistivity, conductivity, molar conductivity and equivalent conductivity.
Solution : Given,
Copyright © 2008-2011. VK Global Publications Pvt. Ltd. All rights reserved. Xamidea,
the Xamidea logo, and VK Global product names are trademarks of VK Global Publications
Pvt. Ltd. or its subsidiaries.
of Use | Login