How many following metals evolve NO (Nitric oxide) gas with dil. `HNO_(3)` (20%)?
`Hg,Cu,Pb,Zn,Fe,Al,Ag,Au,Mn`

Explanation

To determine how many of the given metals evolve nitric oxide (NO) gas when reacted with dilute HNO₃ (20%), we will analyze each metal based on its position in the reactivity series and the known reactions with dilute nitric acid.

### Step-by-Step Solution:

1. **Identify the Metals**: The metals provided are:
- Hg (Mercury)
- Cu (Copper)
- Pb (Lead)
- Zn (Zinc)
- Fe (Iron)
- Al (Aluminum)
- Ag (Silver)
- Au (Gold)
- Mn (Manganese)

2. **Understand the Reactivity Series**: In the reactivity series, metals that are below hydrogen can react with dilute nitric acid to produce nitric oxide (NO). Metals above hydrogen typically do not produce NO but may produce hydrogen gas or other compounds.

3. **Analyze Each Metal**:
- **Hg (Mercury)**: Below hydrogen in the reactivity series. Reacts with dilute HNO₃ to produce NO.
- **Cu (Copper)**: Below hydrogen. Reacts with dilute HNO₃ to produce NO.
- **Pb (Lead)**: Below hydrogen. Reacts with dilute HNO₃ to produce NO.
- **Zn (Zinc)**: Above hydrogen. Does not produce NO; instead, it produces hydrogen gas.
- **Fe (Iron)**: Above hydrogen. With 20% HNO₃, it produces NO₂ (not NO).
- **Al (Aluminum)**: Above hydrogen. Does not produce NO; it reacts differently.
- **Ag (Silver)**: Below hydrogen. Reacts with dilute HNO₃ to produce NO.
- **Au (Gold)**: Below hydrogen. However, it does not produce NO when reacted with dilute HNO₃.
- **Mn (Manganese)**: Above hydrogen. Does not produce NO; it reacts differently.

4. **Count the Metals Producing NO**: From the analysis, the metals that produce NO when reacted with dilute HNO₃ are:
- Hg
- Cu
- Pb
- Ag

5. **Final Count**: Therefore, the total number of metals that evolve NO gas with dilute HNO₃ (20%) is **4**.

### Conclusion:
The answer to the question is **4 metals** (Hg, Cu, Pb, Ag) evolve NO gas with dilute HNO₃.

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