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Variation Of Conductance With Concentration

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Variation Of Conductance With Concentration - Lesson Summary

The conductivity and molar conductivity of an electrolyte change with the concentration of the electrolyte.

As concentration decreases, conductivity decreases for both strong and weak electrolytes.

But molar conductivity increases as concentration decreases. The variation is different for strong electrolytes and weak electrolytes.

Limiting molar conductivity:

This is the conductivity of a solution as its electrolyte concentration approaches zero. Limiting molar conductivity is given the symbol Λ0m.

The molar conductivity of strong electrolytes is weakly dependent on concentration.

A graph of molar conductivity against the square root of concentration:

The y intercept is the limiting molar concentration. The slope of the resulting line is the Kohlrausch coefficient, which depends on the specific salt.

The Kohlrausch coefficient depends on the cation and anion charges of the ions formed by the electrolyte.

The molar conductivity of a weak electrolyte depends greatly on the concentration of the solution. This is because weak electrolytes have a low degree of dissociation as concentration increases.

Since the relationship isn't linear, we can't determine the limiting molar conductivity by extrapolating back to zero concentration. At infinite dilution, the electrolyte dissociates completely, but the conductivity is so low that it can't be measured directly. Instead, we can use Kohlrausch's law to find the limiting molar concentration.


Degree of dissociation α:

α for a weak electrolyte

Degree of Dissociation α = Am(Molar Conductivity)/A°m(Limiting Molar Conductivity)

If the molar conductivity and the limiting molar conductivity known, then the dissociation constants for weak electrolytes at a given concentration can be find.

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