The following are acid-base conjugate pairs except:

Options

A. `C_(6)H_(5)CO_(2)H`and `C_(6)H_(5)CO_(2)^(-)`
B. `H_(3)O^(+)` and `OH^(-)`
C. `CH_(3) NH_(3)^(+)` and `CH_(3)NH_(2)`
D. `HS^(-)` and` S^(2-)`

Answer

To solve the question regarding which of the following are not acid-base conjugate pairs, we will analyze each option provided.

### Step-by-Step Solution:

1. **Understand Acid-Base Conjugate Pairs**:
- An acid donates a proton (H⁺) and becomes its conjugate base.
- A base accepts a proton and becomes its conjugate acid.

2. **Analyze Each Option**:
- **Option 1: C₆H₅COOH and C₆H₅COO⁻**:
- C₆H₅COOH (benzoic acid) is an acid. When it donates a proton, it becomes C₆H₅COO⁻ (benzoate ion), which is its conjugate base. This is a valid conjugate acid-base pair.

- **Option 2: H₃O⁺ and OH⁻**:
- H₃O⁺ (hydronium ion) is an acid that donates a proton to become H₂O (water). OH⁻ (hydroxide ion) is a base that accepts a proton to also become H₂O. However, H₃O⁺ and OH⁻ do not form a conjugate acid-base pair because they do not directly convert into one another. Therefore, this option is not a valid conjugate pair.

- **Option 3: CH₃NH₃⁺ and CH₃NH₂**:
- CH₃NH₃⁺ (methylammonium ion) is an acid that donates a proton to become CH₃NH₂ (methylamine), which is its conjugate base. This is a valid conjugate acid-base pair.

- **Option 4: H₂S and HS⁻**:
- H₂S (hydrogen sulfide) can donate a proton to become HS⁻ (hydrosulfide ion), which is its conjugate base. This is also a valid conjugate acid-base pair.

3. **Conclusion**:
- The only option that does not represent a conjugate acid-base pair is **Option 2: H₃O⁺ and OH⁻**.

### Final Answer:
The following are acid-base conjugate pairs except: **Option 2: H₃O⁺ and OH⁻**.