Question 7:Define conductivity and molar conductivity for the solution of an electrolyte. Discuss their variation with concentration.
Conductivity of a solution is equal to the conductance of a solution of 1 cm length and cross section area of 1 square cm. it may also be define as the conductance of ine centimeter cube of the conductor . It is represented by the symbol Kappa (κ). mathematically we can write
κ = 1/ p
here ρ is resistivity
the unit of K is ohm –1 cm –1 or S cm–1
The conductivity, κ, of an electrolytic solution depends on the concentration of the electrolyte, nature of solvent and temperature.
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,
Distance is unit so l = 1
Volume = area of base × length
So V = A × 1 = A
Λm = κV
Molar conductivity increases with decrease in concentration. When concentration approaches zero, the molar conductivity is known as limiting molar conductivity and is represented by the symbol Ë°m.
For strong electrolytes, Λ increases slowly with dilution and can be represented by the equation:
Λm = Ë°m° – A c ½
It can be seen that if we plot Λm against c1/2, we obtain a straight line with intercept equal to Ëm° and slope equal to ‘–A’. The value of the constant ‘A’ for a given solvent and temperature depends on the type of electrolyte i.e., the charges on the cation and anion produced on the dissociation of the electrolyte in solution.