Which of the following cannot evolve `H_(2)` from dil acid?

Options

A. Pt
B. Zn
C. Mg
D. Pb

Answer

To determine which metal cannot evolve hydrogen gas (H₂) from a dilute acid, we need to analyze the standard reduction potentials (E°) of the metals in question. The metals mentioned are Platinum (Pt), Zinc (Zn), Magnesium (Mg), and Lead (Pb).

### Step-by-Step Solution:

1. **Understanding Standard Reduction Potentials**:
- The standard reduction potential (E°) indicates how easily a species can be reduced. A higher (more positive) E° means a stronger oxidizing agent, which means it is less likely to displace hydrogen from acids.
- The standard hydrogen electrode (SHE) has an E° of 0 V, which serves as a reference point.

2. **Identifying the E° Values**:
- Platinum (Pt): E° = +1.188 V
- Lead (Pb): E° = -0.13 V
- Zinc (Zn): E° = -0.76 V
- Magnesium (Mg): E° = -2.37 V

3. **Comparing E° Values**:
- We compare the E° values of the metals with that of hydrogen (0 V).
- Metals with E° values lower than 0 V can displace hydrogen from dilute acids, while those with E° values higher than 0 V cannot.

4. **Analysis**:
- Platinum has the highest E° (+1.188 V), which is greater than 0 V. Therefore, it cannot displace hydrogen from dilute acid.
- Lead, Zinc, and Magnesium all have negative E° values, which means they can displace hydrogen and evolve H₂.

5. **Conclusion**:
- The metal that cannot evolve hydrogen gas from dilute acid is **Platinum (Pt)**.

### Final Answer:
**Platinum (Pt)** cannot evolve H₂ from dilute acid.