Consider the following species:
(i) `CH_(3)^(+)`
(ii) `(C_(3)H_(5))_(3)Al`
(iii) `HCHO`
(iv) `CH_(4)`
(v) `(C_(2)H_(5))_(3)N`
(vI) `TiCl_(4)`
(vii) `CO_(2)`
(viii) `SiCl_(4)`
(ix) `BF_(3)`
the find out total number of species which can act as Lewis acid.

Explanation

To determine the total number of species that can act as Lewis acids among the given options, we need to analyze each species based on the definition of a Lewis acid. A Lewis acid is defined as a species that can accept an electron pair, which typically means it has vacant orbitals or is positively charged.

Let's evaluate each species step by step:

1. **CH₃⁺ (Methyl Cation)**:
- This species is a positively charged ion (cation). Cations can accept electron pairs due to their positive charge.
- **Conclusion**: Acts as a Lewis acid.

2. **(C₃H₅)₃Al (Tri-n-propylaluminum)**:
- Aluminum in this compound has an incomplete octet and can accept electron pairs.
- **Conclusion**: Acts as a Lewis acid.

3. **HCHO (Formaldehyde)**:
- The carbon atom in formaldehyde has a partial positive charge and can accept electron pairs.
- **Conclusion**: Acts as a Lewis acid.

4. **CH₄ (Methane)**:
- Methane has a complete octet and does not have the ability to accept electron pairs.
- **Conclusion**: Does not act as a Lewis acid.

5. **(C₂H₅)₃N (Triethylamine)**:
- Although nitrogen can donate a lone pair, it does not have vacant orbitals to accept electron pairs.
- **Conclusion**: Does not act as a Lewis acid.

6. **TiCl₄ (Titanium Tetrachloride)**:
- Titanium has vacant d-orbitals and can accept electron pairs.
- **Conclusion**: Acts as a Lewis acid.

7. **CO₂ (Carbon Dioxide)**:
- Carbon dioxide can act as a Lewis acid because the carbon atom can accept electron pairs.
- **Conclusion**: Acts as a Lewis acid.

8. **SiCl₄ (Silicon Tetrachloride)**:
- Silicon can accept electron pairs due to its vacant d-orbitals.
- **Conclusion**: Acts as a Lewis acid.

9. **BF₃ (Boron Trifluoride)**:
- Boron has an incomplete octet and can accept electron pairs.
- **Conclusion**: Acts as a Lewis acid.

Now, let's summarize the findings:

- **Species that act as Lewis acids**:
1. CH₃⁺
2. (C₃H₅)₃Al
3. HCHO
4. TiCl₄
5. CO₂
6. SiCl₄
7. BF₃

- **Total number of species that can act as Lewis acids**: **7**