Common ion effect - Practice Questions & MCQ

The value of the degree of dissociation for a weak electrolyte is decreased by the addition of a strong electrolyte having a common ion. As a result of this effect, the concentration of the uncommon ion of the weak electrolyte decreases.
For example:

•  
  weak electrolyte
  
   
                     Common ion
   Here  for NH₄OH will be decreased by NH₄Cl
• 
  
  Here  for CH₃COOH will be decreased by CH₃COONa

Applications of Common ion effect

• The solubility of a partially soluble salt decreases due to the common ion effect. For example, the presence of AgNO₃ or KCl decreases the solubility of AgCl in water.
• Salting out of soap by addition of NaCl.
• Purification of NaCl by passing HCl gas.

Isohydric Solution: These are the solutions having the same concentration of common ions.

The pH of a mixture of a weak acid and a strong acid can be understood using the following example.

Weak acid: H₂S (0.1M)
Strong acid: HCl (0.3M)
The K_a value for this mixture = 1.2 x 10^-20

The chemical equation for H₂S is given as follows:

           
Initial:     c              0              0
Equil:     c - c?        c?            c?

Thus, the equilibrium constant K_a is given as follows:

Again, the chemical equation for HCl is given as follows:
         
Initial: 0.3            0            0
Equil:   0            0.3         0.3

Thus, total [H⁺] for the mixture = c? + 0.3
                                       [H⁺] = 3 x 10^-1          (? is very small for weak acids)

Now, the pH for the mixture is given as follows: