How many of the following gases/vapours are colourless?
`CO_(2),SO_(2),H_(2)S,NO_(2),HCl,Br_(2),I_(2),CO,Cl_(2)`

Explanation

To determine how many of the given gases/vapours are colorless, we will analyze each gas one by one based on their properties.

1. **CO₂ (Carbon Dioxide)**:
- CO₂ is a colorless gas. It does not absorb visible light due to a high energy gap between its HOMO and LUMO, which prevents electron transitions that would result in color.

2. **SO₂ (Sulfur Dioxide)**:
- SO₂ is also a colorless gas. Similar to CO₂, the energy gap is high, and it does not exhibit color.

3. **H₂S (Hydrogen Sulfide)**:
- H₂S is a colorless gas. It does not have the capability for electron transitions that would impart color.

4. **NO₂ (Nitrogen Dioxide)**:
- NO₂ is a colored gas. It has a significant electron transition capability due to the presence of unpaired electrons, and it appears brown.

5. **HCl (Hydrochloric Acid)**:
- HCl is generally considered a colorless gas. Although it can form white fumes when in contact with moisture, it is classified as colorless.

6. **Br₂ (Bromine)**:
- Br₂ is a colored gas. It appears reddish-brown due to electron transitions.

7. **I₂ (Iodine)**:
- I₂ is also a colored gas. It appears violet due to electron transitions.

8. **CO (Carbon Monoxide)**:
- CO is a colorless gas. It does not exhibit color due to a high energy gap preventing electron transitions.

9. **Cl₂ (Chlorine)**:
- Cl₂ is a colored gas. It appears yellow due to the transitions of electrons.

Now, let's summarize the colorless gases identified:

- CO₂: Colorless
- SO₂: Colorless
- H₂S: Colorless
- HCl: Colorless
- CO: Colorless

The colored gases identified are:

- NO₂: Colored (brown)
- Br₂: Colored (reddish-brown)
- I₂: Colored (violet)
- Cl₂: Colored (yellow)

**Final Count of Colorless Gases**:
There are 5 colorless gases from the list.

### Summary of Colorless Gases:
1. CO₂
2. SO₂
3. H₂S
4. HCl
5. CO

**Total Colorless Gases**: **5**

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