Which combination cannot be used for the preparation of hydrogen gas in the laboratory ?
I. Zinc/conc. `H_(2)SO_(4)`
II. Zinc/ `HNO_(3)`
III. Pure zinc/dil. `H_(2)SO_(4)`

Options

A. I and II
B. I, II, III
C. III only
D. I and III

Answer

To determine which combination cannot be used for the preparation of hydrogen gas in the laboratory, we will analyze each of the given reactions step by step.

### Step 1: Analyze the first reaction - Zinc and concentrated H₂SO₄
- **Reaction**: Zn + H₂SO₄ (conc) → ZnSO₄ + SO₂ + H₂O
- **Explanation**: When zinc reacts with concentrated sulfuric acid, zinc sulfate, sulfur dioxide, and water are produced. Notably, hydrogen gas is **not** produced in this reaction.
- **Conclusion**: This combination **cannot** be used to prepare hydrogen gas.

### Step 2: Analyze the second reaction - Zinc and HNO₃
- **Reaction**: Zn + HNO₃ → Zn(NO₃)₂ + NO + H₂O
- **Explanation**: When zinc reacts with nitric acid, zinc nitrate, nitrogen monoxide (NO), and water are formed. Again, hydrogen gas is **not** produced in this reaction.
- **Conclusion**: This combination **cannot** be used to prepare hydrogen gas.

### Step 3: Analyze the third reaction - Pure zinc and dilute H₂SO₄
- **Reaction**: Zn + H₂SO₄ (dil) → ZnSO₄ + H₂
- **Explanation**: When pure zinc reacts with dilute sulfuric acid, zinc sulfate and hydrogen gas are produced. Here, hydrogen gas is indeed produced.
- **Conclusion**: This combination **can** be used to prepare hydrogen gas.

### Final Conclusion:
From the analysis:
- The first reaction (Zn with concentrated H₂SO₄) does not yield hydrogen.
- The second reaction (Zn with HNO₃) also does not yield hydrogen.
- The third reaction (pure Zn with dilute H₂SO₄) does yield hydrogen.

Thus, the combinations that **cannot** be used for the preparation of hydrogen gas in the laboratory are **I and II**.

### Answer:
**I and II cannot be used for the preparation of hydrogen gas.**

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