Question

Define limiting molar conductivity. Why conductivity of an electrolyte solution decreases with the decrease in concentration?

Answer 1

Limiting molar conductivity:

• The molar conductivity of a solution at infinite dilution is known as limiting molar conductivity.
• In other words, When the concentration of the electrolyte approaches zero, the molar conductivity is known as limiting molar conductivity.

Molar conductivity:

• Molar conductivity (Ʌ_m) of a solution at a given concentration is the conductance of volume “V” of a solution containing one mole of electrolyte kept between two electrodes with an area of cross-section “A” and distance of unit length.
• Mathematically,

${\mathrm{Ʌ}}_{\mathrm{m}}=\mathrm{K}/\mathrm{c}$

Here,

c = concentration in moles per volume
К = specific conductivity
Ʌ_m = molar conductivity

V = Volume of the solution

As the solution contains only one mole of electrolyte, the above equation can be modified as:

${\mathrm{Ʌ}}_{\mathrm{m}}=\mathrm{K}\mathrm{V}$

• With the decrease in concentration, conductivity decreases because the number of ions per unit volume that hold the current in a solution decrease with dilution whereas the molar conductivity of a solution increases with the decrease in concentration.
• This increase in molar conductivity is because of the increase in the total volume containing one mole of the electrolyte.