Question

What is Kohlrausch’s law? Explain any two applications of Kohlrausch’s law.

Answer 1

Hint: Kohlrausch’s law is based on the conductivity of ions i.e. anions, and cations. The applications of this law are also based on the calculation of molar conductivities, and dissociation constant, or we can say that degree of ionisation for an electrolyte. We will explain on the basis of their relation with each other.

Complete step by step solution:
Now, first we will discuss Kohlrausch's law. As mentioned it is relatable with the conductivities of cations, and anions.
So, we can say this law states that the sum of conductance of the anions, and cations will be equal to the value of equivalent conductivity of an electrolyte at infinite dilution.
Now, we know that there is an increase in the concentration with the increase in the value of molar conductivity of a solution.
Thus, we can say that it is related to the volume of the electrolyte, i.e. the volume of one mole of electrolyte will increase.
So, it is defined to be limiting molar conductivity. In other terms we will define limiting molar conductivity that is when the electrolyte concentration approaches the zero value.
Now, we will discuss the applications of Kohlrausch’s law.
The first application we have, i.e. to determine the value of limiting molar conductivities of any electrolyte. If we talk about the weak electrolyte, they will have lower value of molar conductivity.
So, we can say it is used for mainly weak electrolytes.
The second application we have, i.e. to determine the value of dissociation constant. It is related to the molar conductivity, and limiting molar conductivity.
We can represent it as:
$\alpha$ = $\dfrac{\Lambda}{E_m^\circ}$
Here, numerator represents the molar conductivity, and the denominator represents the limiting molar conductivity.
If we explain the whole concept in consideration with the example of acetic acid; then the Kohlrausch’ law for acetic acid will be:
$\Lambda$ $_0$ (CH$_3$COOH) = $\Lambda$ $_0$ (HCl) + $\Lambda$ $_0$ (CH$_3$COONa) - $\Lambda$ $_0$ (NaCl)
The above written equation indicates towards the molar conductivity of acetic acid at the zero concentration value.
In the last we can conclude that it is relatable to the limiting molar conductivity value for a weak electrolyte.

Note: Don’t get confused while writing the applications of Kohlrausch’s law. We said that it is valid for the weak electrolytes because there is a steep increase in molar conductivity, and limiting molar conductivity cannot be attained by zero concentration value.