Which one of the following is not acid-base conjugate pair ?

Options

A. `HONO,NO_2^-`
B. `CH_3NH_3^+,CH_3NH_2`
C. `C_6H_5-COOH,C_6H-5COO^-`
D. `H_3O^+,OH^-`

Answer

To determine which of the following is not an acid-base conjugate pair, we need to analyze each option based on the concept of conjugate acid-base pairs. A conjugate acid-base pair consists of an acid and its corresponding base, which differ by a single proton (H⁺).

### Step-by-Step Solution:

1. **Identify the Options**:
- Option A: H₂O and NO₂⁻
- Option B: CH₃CCH₃NH₃⁺ and CH₃NH₂
- Option C: C₆H₅COOH and C₆H₅COO⁻
- Option D: H₃O⁺ and OH⁻

2. **Analyze Each Option**:
- **Option A**: H₂O (water) can donate a proton (H⁺) to form OH⁻ (hydroxide ion). However, NO₂⁻ (nitrite ion) is not formed from H₂O. Therefore, H₂O and NO₂⁻ do not form a conjugate pair.
- **Option B**: CH₃CCH₃NH₃⁺ (ammonium ion) can lose a proton to form CH₃CCH₃NH₂ (amine). This is a conjugate acid-base pair.
- **Option C**: C₆H₅COOH (benzoic acid) can lose a proton to form C₆H₅COO⁻ (benzoate ion). This is also a conjugate acid-base pair.
- **Option D**: H₃O⁺ (hydronium ion) can lose a proton to form H₂O (water). However, OH⁻ (hydroxide ion) is not the conjugate base of H₃O⁺. Therefore, H₃O⁺ and OH⁻ do not form a conjugate pair.

3. **Conclusion**:
After analyzing all options, we find that:
- Option A (H₂O and NO₂⁻) is not a conjugate acid-base pair.
- Option D (H₃O⁺ and OH⁻) is also not a conjugate acid-base pair.

However, since the question asks for the option that is not a conjugate pair, we conclude that **Option D** is the correct answer.

### Final Answer:
**Option D: H₃O⁺ and OH⁻ is not an acid-base conjugate pair.**

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