Which of the following do not form conjugate acid - base pair in the right order :-

Options

A. (a)`NH_(3),NH_(2)`
B. (b)`OH^(-),H_(2)O`
C. (c)`HCO_(3),CO_(3)^(2)`
D. (d)`H_(2)S,HS^(-)`

Answer

To determine which of the given pairs do not form a conjugate acid-base pair in the correct order, we need to understand the concept of conjugate acids and bases. A conjugate acid is formed when a base accepts a proton (H⁺), and it differs from the base by one hydrogen ion.

### Step-by-Step Solution:

1. **Understand Conjugate Acid-Base Pairs**:
- A conjugate acid is formed when a base gains a proton (H⁺).
- The relationship is such that the conjugate acid has one more hydrogen ion than the base.

2. **Analyze Each Option**:
- **Option A: NH₂⁻ and NH₃**
- NH₂⁻ is a base, and when it accepts a proton (H⁺), it becomes NH₃.
- Therefore, NH₃ is the conjugate acid of NH₂⁻.
- This pair is in the correct order.

- **Option B: OH⁻ and H₂O**
- OH⁻ is a base, and when it accepts a proton (H⁺), it becomes H₂O.
- Therefore, H₂O is the conjugate acid of OH⁻.
- However, the order is OH⁻ (base) followed by H₂O (conjugate acid), which is correct. But, we need to check other options.

- **Option C: CO₃²⁻ and HCO₃⁻**
- CO₃²⁻ is a base, and when it accepts a proton (H⁺), it becomes HCO₃⁻.
- Therefore, HCO₃⁻ is the conjugate acid of CO₃²⁻.
- This pair is in the correct order.

- **Option D: HS⁻ and H₂S**
- HS⁻ is a base, and when it accepts a proton (H⁺), it becomes H₂S.
- Therefore, H₂S is the conjugate acid of HS⁻.
- This pair is also in the correct order.

3. **Conclusion**:
- After analyzing all options, we find that all pairs except for Option B (OH⁻ and H₂O) are in the correct order. However, since we are looking for the pair that does not form a conjugate acid-base pair in the right order, we conclude that **Option B is the answer**.

### Final Answer:
**The pair that does not form a conjugate acid-base pair in the right order is Option B: OH⁻ and H₂O.**