Question

How to calculate the limiting molar conductivity of strong electrolytes?

Answer 1

Molar conductance

1. Molar conductance is the conductance of one molar electrolyte solution.
2. It depends on ionic strength or concentration of the solution.
3. Unit - ${\mathrm{Sm}}^2{\mathrm{mol}}^{-1}$

Kohlrausch's law

• Kohlrausch's law states that ‘the limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anions and cations of the electrolyte.'
• The limiting molar conductance of strong electrolyte is calculated by using Kohlrausch's law.
• It depends on the type of electrolyte and the temperature for a given solvent.
• The graph is plotted between $\sqrt{\mathrm{c}}$ (concentration) and molar conductance is given below.

• The red line represents strong electrolyte
• Green line represents molar conductivity for intermediate electrolyte.
• Blue line represents molar conductivity for weak electrolyte.
• Dilution results in the dissociation of more electrolytes into ions and effectively increasing the number of active ions in the solution. Hence, conductivity of solution increases.
• Since, the molar conductivity increases with dilution. On exploiting the graph the the point on y axis represents the limiting value .
• The overall equation is given as:

${\mathrm{\Lambda }}_{\mathrm{m}}={{\mathrm{E}}^{\mathrm{o}}}_{\mathrm{m}}–\mathrm{A}\sqrt{\mathrm{c}}$

• Where E is the intercept, which is limiting molar conductance and A is the slope.
• For weak electrolyte, on dilution the conductivity increases suddenly and it reaches infinite value which cannot be calculated by Kohlrausch's law.

Therefore, the limiting molar conductivity of strong electrolytes is calculated by using Kohlrausch's law.