Among the following in which hydrates `H_(2)O` molecules only occupy the interstitial site

Options

A. `BaCl_(2)2H_(2)O`
B. `[[Ni(H_(2)O)_(6)]^(2+)](NO_(3)^(-))_(2)`
C. `CuSO_(4)5H_(2)O`
D. All of these

Answer

To determine which hydrate has water molecules that only occupy interstitial sites, we need to analyze the options provided and understand the nature of the water molecules in each case.

### Step-by-Step Solution:

1. **Understand Interstitial Water**:
- Interstitial water refers to water molecules that occupy spaces (interstitial sites) in a crystal lattice and are not involved in hydrogen bonding with other molecules.

2. **Analyze the Options**:
- **Option 1: BaCl₂·2H₂O**
- In this hydrate, the water molecules are present in interstitial sites and are not hydrogen bonded to the Ba or Cl ions. This makes it a candidate for our answer.

- **Option 2: CoCl₂·6H₂O**
- In this case, the water molecules are coordinated to the cobalt ion, meaning they are involved in coordinate bonding and thus are not just occupying interstitial sites. This option is not correct.

- **Option 3: CuSO₄·5H₂O**
- Here, four of the five water molecules are coordinated to the copper ion, while one water molecule is hydrogen bonded. Therefore, this option does not fit our criteria.

- **Option 4: MgSO₄·7H₂O**
- Similar to the previous options, the water molecules in this hydrate are involved in hydrogen bonding and coordination with the magnesium ion, making it incorrect for our requirement.

3. **Conclusion**:
- The only option where water molecules occupy interstitial sites without hydrogen bonding is **BaCl₂·2H₂O**.

### Final Answer:
The correct option is **BaCl₂·2H₂O**.