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__Conductance__

**Introduction: **

Both metallic and electrolytic conductors obey Ohm's law

i.e. V = IR

where V = Potential difference in volt; I Current in ampere ; R = resistance in Ohm

We know, resistance is directly proportional to length of conductor and inversely proportional to cross sectional area of the conductor.

Specific resistance is the resistance of a conductor having lengths of 1 cm and cross sectional area of 1 cm^{2}.

Unit of R is ohm and unit of specific resistance is ohm cm

**Reciprocal of resistance is called as conductance and reciprocal of specific resistance is called as specific conductance. **

where C = conductance ohm^{-1}; K = specific conductance ohm^{-1} cm^{-1}.

Ohm and siemens are other units of conductance.

Specific conductance = Cell constant x Conductance.

**Specific Conductance is Conductance of 1 cm ^{3} of an electrolyte solution.**

In case of electrolytic solution, the specific conductance is defined as the conductance of a solution of definite concentration enclosed in a cell having two electrodes of unit area separated by 1 cm apart.

**1. Equivalent Conductance **

Equivalent conductance is the conductance of an electrolyte solution containing 1 gm equivalent of electrolyte. It is denoted by ^.

^ = K Ã— V

( ^ = ohm^{-1} cm^{-1} Ã— cm^{3} = ohm^{-1} cm^{2})

Usually concern ration of electrolyte solution is expresses as C gm equivalent per litre.

Thus, V =

{Volume having 1 gm equivalent electrolyte in the solution} Thus, ^ = K Ã— .

**2. Molar Conductance**

Molar conductance may be defined as conductance of an electrolyte solution having 1 gm mole electrolyte in a litre. It is denoted by ^_{m}.

^_{m} = K Ã— V

Usually concentration of electrolyte solution is expressed as 'M' gm mole electrolyte per litre.

Thus, V =

Hence, ^_{m} = K Ã—

**Relation between ^ and ^ _{m} : **^

**Determination of ^ _{m}^{0} or ^**

A plot of ^_{m} vs as found experimentally is as shown below graphically.

the ^_{m} vs plot of strong electrolyte being linear it can be extrapolated to zero concentration.

Thus, ^_{m} values of the solution of the test electrolyte are determined at various concentrations the concentrations should be as low as good.

^_{m} values are then plotted against when a straight line is obtained. This is the extrapolated to zero concentration. The point where the straight line intersects ^_{m} axis is ^^{Âº}_{m} of the strong electrolyte.

However, the plot in the case weak electrolyte being non linear, shooting up suddenly at some low concentration and assuming the shape of a straight line parallel to ^_{m} axis. Hence extrapolation in this case is not possible. Thus, ^_{0} of a weak electrolyte cannot be determined experimentally. It can, however, be done with the help of Kohlrausch's law to be discussed later.

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