Question

The degree of dissociation of weak electrolyte increases by:
(A) Increasing the concentration of weak electrolyte solution
(B) Adding a common ion
(C) Dissolution in solvent of low dielectric constant
(D) Adding more amount of solvent into the solution.

Answer 1

Hint: The degree of dissociation of a weak electrolyte is directly proportional to the square root of dilution because on dilution, it dissociates into ions due to the high dielectric constant of water or solvent and ions are readily available thereby increasing the degree of dissociation.

Complete step-by-step answer:
Ostwald’s dilution law is used to describe the dissociation constant of the weak electrolyte with the degree of dissociation (\[\alpha \]) and the concentration of the weak electrolyte \[(C)\]
Let us take a binary electrolyte AB which dissociates into \[{A^ + }\] and \[{B^ - }\] ions.
Such that, \[AB \rightleftharpoons {A^ + } + {B^ - }\]
For very weak electrolytes, we know that degree of dissociation is very less i.e. \[\alpha \; < < < {\text{ }}1,\]
 Therefore, dissociation constant K will be \[K = C{\alpha ^2}\],
From this we get degree of dissociation of any ion \[\alpha \] \[ = \sqrt {\tfrac{K}{C}} \]
Where, \[K = \] dissociation constant of a weak acid.
\[\alpha = \] degree of dissociation and c is the concentration of ions present.
There are many factors that affect the degree of dissociation. These are nature of electrolyte, nature of solvent, nature of other substances present in solvent, dilution and temperature.
When more amount of solvent such as water is added to the solution, it results in dissociating the molecules into ions of a weak electrolyte. Thus, the degree of dissociation of weak electrolyte increases upon dilution.

Hence, the correct option is (D)..

Note: The Ostwald’s dilution law was proposed in 1888. The Ostwald’s dilution law holds good only for weak electrolytes and fails completely in the case of strong electrolytes because strong electrolytes are completely ionized at all dilution.