Question

Assertion- Acetic acid is more acidic in sodium acetate solution than in NaCl solution.
Reason- Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion.
(A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion
(B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion
(C) Assertion is true but reason is false
(D) Assertion is false but reason is true
(E) Both assertion and reason are false

Answer 1

Hint: Firstly, let’s see what is meant by common ion effect-
Common ion effect – it is the difference in equilibrium concentrations between a solution containing a common ion and the same solution in pure water. A common ion is present both in a salt added to a solution and in the solution itself.

Step by step solution:
-Acetic acid is a weak acid, represented by formula $C{{H}_{3}}COOH$ . Sodium acetate is shown by formula $C{{H}_{3}}COONa$.
Let’s see the reaction
\[\begin{align}
  & C{{H}_{3}}COOH\left( aq \right)\to C{{H}_{3}}CO{{O}^{-}}\left( aq \right)+{{H}^{+}}\left( aq \right) \\
 & C{{H}_{3}}COONa\left( aq \right)\to C{{H}_{3}}CO{{O}^{-}}\left( aq \right)+N{{a}^{+}}\left( aq \right) \\
\end{align}\]
-In above reaction we can see that acetic acid contains an acetate ion and sodium acetate also contains acetate ion .so, as both of the reactions involve the same ion, then one reaction can affect the position of another reaction.
-The ion that appears in both reactions, that is $C{{H}_{3}}CO{{O}^{-}}$ is a common ion.
-When $C{{H}_{3}}COONa$ is added to a solution of $C{{H}_{3}}COOH$,the concentration of $C{{H}_{3}}CO{{O}^{-}}$increases. The concentration of ${{H}^{+}}$ ions depends upon the concentration of $C{{H}_{3}}CO{{O}^{-}}$. When concentration of $C{{H}_{3}}CO{{O}^{-}}$ increases, the concentration of ${{H}^{+}}$decreases. This means dissociation of $C{{H}_{3}}COOH$decreases.

Therefore, the correct option is (D), we can say that assertion is false but reason is true.

Additional information:
-Common ion effect solubility- if you have a solution as well as solute in equilibrium, then adding a common ion that is (an ion that is common with the dissolving solid) decreases the solubility of the solubility of the solute. This happens because Le Chatelier’s principle states that the reaction will shift towards the left (towards the reactants) to relieve the stress of excess product.
-Common ion effect on precipitation – when we add an additional amount of ions it leads to increased precipitation of salt.

Note:
-There are some other examples of common ion effect as well like-
  -dissociation of hydrogen sulphide in presence of HCl.
  -solubility of barium iodate in presence of barium nitrate.
-The relation between common ion effect and degree of dissociation is –
\[common\,ion\,effect\,\propto \dfrac{1}{degree\,of\,dissociation}\]
Hence , as the common ion effect increases ,the degree of dissociation decreases.