In which of the following compounds, carbon exhibits a valency of
four but oxidation state of -2 ?

Options

A. `CH_(3)Cl`
B. `CHCl_(3)`
C. `CH_(2)Cl_(2)`
D. `HCHO`

Explanation

To determine in which compound carbon exhibits a valency of four but an oxidation state of -2, we can analyze each compound provided in the question.

### Step-by-Step Solution:

1. **Understanding Valency and Oxidation State**:
- **Valency** refers to the number of bonds an atom can form, while **oxidation state** indicates the degree of oxidation of an atom in a compound, which can be positive, negative, or zero.
- For carbon, a valency of 4 means it can form four bonds.

2. **Analyzing Each Compound**:
- We will analyze the oxidation state of carbon in each compound listed.

3. **Compound 1: CH₃Cl**:
- Structure: H₃C-Cl
- Oxidation state calculation:
\[ X + 3(+1) + (-1) = 0 \implies X + 3 - 1 = 0 \implies X + 2 = 0 \implies X = -2 \]
- Valency: 4 (4 bonds formed: 3 with H and 1 with Cl)
- Oxidation state: -2

4. **Compound 2: CHCl₃**:
- Structure: H-C(Cl)₃
- Oxidation state calculation:
\[ X + 1 + 3(-1) = 0 \implies X + 1 - 3 = 0 \implies X - 2 = 0 \implies X = +2 \]
- Valency: 4 (4 bonds formed: 1 with H and 3 with Cl)
- Oxidation state: +2

5. **Compound 3: CH₂Cl₂**:
- Structure: H₂C(Cl)₂
- Oxidation state calculation:
\[ X + 2(+1) + 2(-1) = 0 \implies X + 2 - 2 = 0 \implies X = 0 \]
- Valency: 4 (4 bonds formed: 2 with H and 2 with Cl)
- Oxidation state: 0

6. **Compound 4: HCHO (Formaldehyde)**:
- Structure: H₂C=O
- Oxidation state calculation:
\[ X + 2(+1) + (-2) = 0 \implies X + 2 - 2 = 0 \implies X = 0 \]
- Valency: 4 (4 bonds formed: 2 with H and 2 with O, one double bond)
- Oxidation state: 0

7. **Conclusion**:
- The only compound where carbon has a valency of 4 and an oxidation state of -2 is **CH₃Cl**.

### Final Answer:
The compound in which carbon exhibits a valency of four but an oxidation state of -2 is **CH₃Cl**.