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Electrochemistry

Question
CBSEENCH12006118

Draw curves to show how the molar conductance of strong electrolytes varies with dilution.

Solution
When a plot is drawn between the molar conductivity versus square root of concentration for strong electrolytes, a curve (nearly straight line) shown in the given Fig is obtained. It is clear from the figure that molar conductance has higher value and increases linearly. Slight increase in the value of Λwith dilution is due to the decrease in interionic attraction with dilution i.e., decrease in concentration.

The molar conductivity at infinite dilution (Λm) can be obtained by extrapolating the above graph of strong electrolytes to zero concentration.
Kohlrausch Law of Independent migration of ions: The law states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte. Thus, if λ°Na+ and λCl are molar limiting conductivity of the sodium and chloride ions respectively, then the limiting molar conductivity for sodium chloride is given by the equation:
Λ°NaCl = λ°Na + λCl
In general, if an electrolyte on dissociation gives v+ cations and v anions then its limiting molar conductivity is given by
Λ° = v + λ°+ + V λ°
Here, λ°+ and λ° are the limiting molar conductivities of the cation and anion respectively.
Kohlrausch’s law helps us to calculate
(i) Determination of molar conductivities of weak electrolytes at infinite dilution.
(ii) Determination of the degree of dissociation of electrolytes.
Degree of dissociation: It is ratio of molar conductivity at a specific countraction ‘C’ to the molar conductivity at infinite dilution. It is denoted by α.
i.e.,                   α = AmCAm