Which of the following is a conjugated acid - base pair?

Options

A. `HCl, NaOH`
B. `NH_(4)Cl,NH_(4)OH`
C. `H_(2)SO_(4), HSO_(4)^(-)`
D. `KCN,HCN`

Answer

To determine which of the given options represents a conjugate acid-base pair, we need to understand the concept of conjugate acids and bases according to the Bronsted-Lowry theory. A conjugate acid-base pair consists of two species that differ by the presence or absence of a proton (H⁺).

### Step-by-Step Solution:

1. **Understand the Definitions**:
- An **acid** is a substance that donates a proton (H⁺).
- A **base** is a substance that accepts a proton (H⁺).
- When an acid donates a proton, it forms its **conjugate base**.
- When a base accepts a proton, it forms its **conjugate acid**.

2. **Analyze Each Option**:
- **Option 1: HCl and NaOH**:
- HCl is an acid that donates H⁺ to form Cl⁻ (its conjugate base).
- NaOH is a base that provides OH⁻ but does not form a conjugate acid from HCl.
- Therefore, this is not a conjugate acid-base pair.

- **Option 2: NH4Cl**:
- NH4⁺ (ammonium ion) can donate a proton to form NH3 (ammonia), making NH4⁺ a conjugate acid and NH3 its conjugate base.
- However, NH4Cl itself is a salt and does not represent a conjugate acid-base pair directly.
- So, this is not a conjugate acid-base pair.

- **Option 3: H2SO4 and HSO4⁻**:
- H2SO4 (sulfuric acid) can donate a proton to form HSO4⁻ (bisulfate ion).
- Here, H2SO4 is the acid, and HSO4⁻ is its conjugate base.
- This is indeed a conjugate acid-base pair.

- **Option 4: KCN and HCN**:
- HCN (hydrogen cyanide) can donate a proton to form CN⁻ (cyanide ion), making HCN the acid and CN⁻ its conjugate base.
- KCN is a salt and does not represent a conjugate acid-base pair with HCN.
- Therefore, this is not a conjugate acid-base pair.

3. **Conclusion**:
- The only option that represents a conjugate acid-base pair is **Option 3: H2SO4 and HSO4⁻**.

### Final Answer:
**The conjugate acid-base pair is H2SO4 and HSO4⁻.**