Which of the following behaves as Lewis acid and not as Bronsted acid?

Options

A. HCl
B. `H_2SO_4`
C. `HSO_3^-`
D. `SO_3`

Answer

To determine which of the following behaves as a Lewis acid and not as a Brønsted acid, we first need to understand the definitions of both types of acids.

### Step-by-Step Solution:

1. **Understanding Lewis Acids and Brønsted Acids**:
- A **Lewis acid** is defined as a substance that can accept a pair of electrons (lone pair) to form a covalent bond.
- A **Brønsted acid** is defined as a substance that can donate a proton (H⁺ ion).

2. **Identifying the Acids**:
- We need to analyze the given options (though not provided in the question, we will assume common examples such as HCl, H₂SO₄, SO₃, etc.) to see which can act as a Lewis acid but not as a Brønsted acid.

3. **Analyzing Common Acids**:
- **HCl**: This can donate H⁺, thus it is a Brønsted acid.
- **H₂SO₄**: This can also donate H⁺ ions, qualifying it as a Brønsted acid.
- **SO₃**: This does not have any H⁺ to donate, so it cannot act as a Brønsted acid. However, it has vacant d orbitals that allow it to accept lone pairs, making it a Lewis acid.

4. **Conclusion**:
- Among the acids analyzed, **SO₃** behaves as a Lewis acid because it can accept a lone pair of electrons but does not behave as a Brønsted acid since it does not donate H⁺ ions.

### Final Answer:
**SO₃** behaves as a Lewis acid and not as a Brønsted acid.