Which pair will show common ion effect ?

Options

A. `BaCI_2 +Ba(NO_3)_2`
B. `NaCI+HCI`
C. `NH_4OH+NH_4CI`
D. `NaCN +KCN`

Answer

To determine which pair will show the common ion effect, we need to analyze the provided pairs of compounds and identify if they consist of a weak electrolyte and a strong electrolyte. The common ion effect occurs when a weak electrolyte is in equilibrium with a strong electrolyte that shares a common ion.

### Step-by-Step Solution:

1. **Identify the Electrolytes**:
- Look at the pairs provided in the question. We need to identify which of them includes a weak electrolyte and a strong electrolyte.

2. **Analyze Each Pair**:
- **Pair 1: BaCl2 and BaNa3O2**:
- Both BaCl2 and BaNa3O2 are strong electrolytes. They completely dissociate in solution, so they do not exhibit the common ion effect.
- **Pair 2: NiCl and HCl**:
- Both NiCl and HCl are strong electrolytes. Again, they do not exhibit the common ion effect.
- **Pair 3: NH4OH and NH4Cl**:
- NH4OH is a weak base (weak electrolyte), and NH4Cl is a strong electrolyte. The common ion here is NH4+. The presence of NH4Cl will suppress the dissociation of NH4OH, demonstrating the common ion effect.
- **Pair 4: NaCN and KCN**:
- Both NaCN and KCN are strong electrolytes. They do not exhibit the common ion effect.

3. **Conclusion**:
- The only pair that shows the common ion effect is **NH4OH and NH4Cl**.

### Final Answer:
The pair that will show the common ion effect is **NH4OH and NH4Cl**.