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 nitric acid (HNO3, 20%), we need to analyze each metal based on its reactivity in relation to hydrogen in the electrochemical series.

### Step-by-Step Solution:

1. **Understand the Reactivity Series**: Metals that are less reactive than hydrogen will react with dilute nitric acid to produce nitric oxide (NO) gas. Metals that are more reactive than hydrogen will not produce NO; instead, they may produce hydrogen gas or other nitrogen oxides.

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

3. **Evaluate Each Metal**:
- **Hg (Mercury)**: Less reactive than hydrogen. **Produces NO**.
- **Cu (Copper)**: Less reactive than hydrogen. **Produces NO**.
- **Pb (Lead)**: Less reactive than hydrogen. **Produces NO**.
- **Zn (Zinc)**: More reactive than hydrogen. **Does not produce NO** (produces H2).
- **Fe (Iron)**: More reactive than hydrogen. **Does not produce NO** (produces NO2 and O2).
- **Al (Aluminum)**: More reactive than hydrogen. **Does not produce NO** (produces NO2).
- **Ag (Silver)**: Less reactive than hydrogen. **Produces NO**.
- **Au (Gold)**: Less reactive than hydrogen but very unreactive. **Does not produce NO** (does not react with dilute HNO3).
- **Mn (Manganese)**: More reactive than hydrogen. **Does not produce NO** (produces H2).

4. **Count the Metals that Produce NO**: From the evaluation, the metals that produce nitric oxide (NO) are:
- Hg
- Cu
- Pb
- Ag

5. **Final Count**: There are **4 metals** that evolve nitric oxide gas when reacted with dilute HNO3.

### Conclusion:
The number of metals that evolve NO gas with dilute HNO3 (20%) is **4**.

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