JEE Chemistry 2025 Question with Solution

Given: We need to identify the electrolyte used to obtain $\text{H}_2\text{S}_2\text{O}_8$ by electrolysis.

Find: The correct option among the given electrolytes.

Peroxydisulfuric acid is formed during the electrolysis of concentrated sulphuric acid.

The relevant anodic process is:

$$2\,\text{HSO}_4^-\,\text{(aq)} \rightarrow \text{H}_2\text{S}_2\text{O}_8\,\text{(aq)} + 2e^-$$

This requires a high concentration of $\text{HSO}_4^-$ ions, which is provided by concentrated sulphuric acid.

• Dilute sodium sulphate solution is not suitable.
• Dilute sulphuric acid is not suitable because the concentration is low.
• Acidified dilute sodium sulphate is also not suitable because adequate $\text{HSO}_4^-$ concentration is not available.

Therefore, the correct option is C.

Given: Formation of $\text{H}_2\text{S}_2\text{O}_8$ by electrolysis.

Find: Which electrolyte makes this formation possible.

Electrolyte nature controls the product formed during electrolysis. For peroxydisulfuric acid formation, concentrated sulphuric acid is specifically required.

The formation step is:

$$2\,\text{HSO}_4^-\,\text{(aq)} \rightarrow \text{H}_2\text{S}_2\text{O}_8\,\text{(aq)} + 2e^-$$

Now examine the options:

1. Dilute solution of sodium sulphate: not suitable because it does not provide the required concentrated bisulphate medium.
2. Dilute solution of sulphuric acid: not suitable because the concentration is too low.
3. Concentrated solution of sulphuric acid: suitable because it provides a high concentration of $\text{HSO}_4^-$ ions.
4. Acidified dilute solution of sodium sulphate: not suitable because the medium is still dilute and does not adequately support formation of $\text{H}_2\text{S}_2\text{O}_8$.

Therefore, the electrolyte used is concentrated solution of sulphuric acid, so the correct option is C.