Which one of the following metals can not be obtained on electrolysis of aqueous solution of its salts?

Options

A. (a) Mg
B. (b) Ag
C. (c) Cu
D. (d) Cr

Answer

To determine which metal cannot be obtained through electrolysis of its aqueous salt solution, we need to analyze the standard reduction potentials of the given metals. The metal with a lower (more negative) standard reduction potential than hydrogen ions (H⁺) will not be deposited at the cathode during electrolysis.

### Step-by-Step Solution:

1. **Understand Electrolysis**: In electrolysis, metals are deposited at the cathode where reduction occurs. The metal ions in solution gain electrons to form solid metal.

2. **Identify the Standard Reduction Potentials**: We need to look up the standard reduction potentials (E°) for the metal ions given in the options. The standard reduction potential for the reduction of H⁺ to H₂ is 0 V.

3. **List the Metals and Their Reduction Reactions**:
- **Magnesium (Mg)**:
\[ \text{Mg}^{2+} + 2e^- \rightarrow \text{Mg} \quad (E° = -2.36 \, \text{V}) \]
- **Silver (Ag)**:
\[ \text{Ag}^+ + e^- \rightarrow \text{Ag} \quad (E° = +0.80 \, \text{V}) \]
- **Copper (Cu)**:
\[ \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \quad (E° = +0.34 \, \text{V}) \]
- **Chromium (Cr)**:
\[ \text{Cr}^{3+} + 3e^- \rightarrow \text{Cr} \quad (E° = -0.74 \, \text{V}) \]

4. **Compare the Standard Reduction Potentials**:
- The reduction potential for H⁺ to H₂ is 0 V.
- **Mg**: -2.36 V (cannot be reduced, more negative than 0 V)
- **Ag**: +0.80 V (can be reduced, more positive than 0 V)
- **Cu**: +0.34 V (can be reduced, more positive than 0 V)
- **Cr**: -0.74 V (cannot be reduced, more negative than 0 V)

5. **Conclusion**: The metals that cannot be obtained through electrolysis of their aqueous salts are those with standard reduction potentials more negative than 0 V. In this case, both Magnesium (Mg) and Chromium (Cr) cannot be obtained. However, since the question asks for one metal, we can conclude that **Magnesium (Mg)** is the most prominent example.

### Final Answer:
**Magnesium (Mg)** cannot be obtained on electrolysis of aqueous solution of its salts.