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 determine which of the given options are not acid-base conjugate pairs, we need to understand the concept of conjugate acid-base pairs. A conjugate acid-base pair consists of two species that differ by a single proton (H⁺). The acid donates a proton to become its conjugate base, and the base accepts a proton to become its conjugate acid.

### Step-by-Step Solution:

1. **Identify the pairs:**
- **Option 1:** C₆H₅CO₂H and C₆H₅CO₂⁻
- **Option 2:** H₃O⁺ and OH⁻
- **Option 3:** CH₃NH₃⁺ and CH₃NH₂
- **Option 4:** HS⁻ and S²⁻

2. **Analyze each pair:**
- **Option 1:** C₆H₅CO₂H (benzoic acid) donates a proton to become C₆H₅CO₂⁻ (benzoate ion). They differ by one proton, so this is a conjugate acid-base pair.
- **Option 2:** H₃O⁺ (hydronium ion) and OH⁻ (hydroxide ion) differ by two protons. H₃O⁺ can donate a proton to become OH⁻, but OH⁻ cannot accept a proton to become H₃O⁺ in the context of being a conjugate pair. Hence, they do not form a conjugate acid-base pair.
- **Option 3:** CH₃NH₃⁺ (methylammonium ion) donates a proton to become CH₃NH₂ (methylamine). They differ by one proton, so this is a conjugate acid-base pair.
- **Option 4:** HS⁻ (hydrosulfide ion) donates a proton to become S²⁻ (sulfide ion). They differ by one proton, so this is a conjugate acid-base pair.

3. **Conclusion:**
The only pair that does not qualify as a conjugate acid-base pair is **Option 2: H₃O⁺ and OH⁻** because they differ by two protons.

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

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