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 options is not an acid-base conjugate pair, we need to analyze each option provided. An acid-base conjugate pair consists of an acid and its corresponding base, which is formed when the acid donates a proton (H⁺).

### Step-by-Step Solution:

1. **Identify the Acid-Base Pairs**:
- An acid (HA) donates a proton (H⁺) to form its conjugate base (A⁻).
- Conversely, a base (B) can accept a proton to form its conjugate acid (BH⁺).

2. **Examine Each Option**:
- **Option 1: HONO (Nitrous Acid)**
- When HONO loses H⁺, it forms NO₂⁻ (Nitrite ion).
- Thus, the pair is (HONO, NO₂⁻) - this is a conjugate acid-base pair.

- **Option 2: CH₃NH₃⁺ (Methylammonium Ion)**
- When CH₃NH₃⁺ loses H⁺, it forms CH₃NH₂ (Methylamine).
- Thus, the pair is (CH₃NH₃⁺, CH₃NH₂) - this is a conjugate acid-base pair.

- **Option 3: C₆H₅COOH (Benzoic Acid)**
- When C₆H₅COOH loses H⁺, it forms C₆H₅COO⁻ (Benzoate ion).
- Thus, the pair is (C₆H₅COOH, C₆H₅COO⁻) - this is a conjugate acid-base pair.

- **Option 4: H₃O⁺ (Hydronium Ion)**
- When H₃O⁺ loses H⁺, it forms H₂O (Water).
- However, H₂O is not a conjugate base of H₃O⁺ in the context of acid-base pairs; rather, it is the product of the proton donation. OH⁻ is considered a base, but it does not form a conjugate pair with H₃O⁺ as it is not the direct product of H₃O⁺ losing H⁺.

3. **Conclusion**:
- The only option that does not form a proper acid-base conjugate pair is **Option 4: H₃O⁺ and H₂O**.

### Final Answer:
**Option 4: H₃O⁺ and H₂O is not an acid-base conjugate pair.**

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