Find the total number of metals, which makes a thin protective layer of its oxide on treatment with conc. `HNO_(3)`.
`Zn,Cu,Al,Pt,Cr,Au,Pb,Fe`

Explanation

To solve the problem of finding the total number of metals that form a thin protective layer of their oxide when treated with concentrated HNO3, we will analyze each metal provided in the list: Zn, Cu, Al, Pt, Cr, Au, Pb, and Fe.

### Step-by-Step Solution:

1. **Identify the Metals**: The metals given are:
- Zinc (Zn)
- Copper (Cu)
- Aluminium (Al)
- Platinum (Pt)
- Chromium (Cr)
- Gold (Au)
- Lead (Pb)
- Iron (Fe)

2. **Understand the Reaction with Concentrated HNO3**:
- Some metals can react with concentrated nitric acid (HNO3) and some can form a protective oxide layer that prevents further reaction.

3. **Analyze Each Metal**:
- **Zinc (Zn)**: Reacts with concentrated HNO3 and does not form a protective oxide layer.
- **Copper (Cu)**: Reacts with concentrated HNO3 and does not form a protective oxide layer.
- **Aluminium (Al)**: Forms a thin protective layer of Al2O3 and does not react further with concentrated HNO3.
- **Platinum (Pt)**: Does not react with concentrated HNO3 and does not form a protective oxide layer.
- **Chromium (Cr)**: Forms a thin protective layer of CrO3 and does not react further with concentrated HNO3.
- **Gold (Au)**: Does not react with concentrated HNO3 and does not form a protective oxide layer.
- **Lead (Pb)**: Reacts with concentrated HNO3 and does not form a protective oxide layer.
- **Iron (Fe)**: Forms a thin protective layer of Fe2O3 and does not react further with concentrated HNO3.

4. **Count the Metals that Form Protective Oxide Layers**:
- The metals that form a protective oxide layer are:
- Aluminium (Al)
- Chromium (Cr)
- Iron (Fe)

5. **Total Count**:
- Therefore, the total number of metals that form a thin protective layer of their oxide when treated with concentrated HNO3 is 3.

### Final Answer:
The total number of metals that form a thin protective layer of their oxide on treatment with concentrated HNO3 is **3**.

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