Among the following, the number of halide(s) which is/are inert to hydrolysis is `"_____"`
(a)`BF_3`
(b) `SiCl_4`
(c ) `PCl_5`
(d) `SF_6`

Answer

To determine the number of halides that are inert to hydrolysis from the given options, we need to analyze each compound's ability to react with water. Hydrolysis generally involves the reaction of a compound with water, where water acts as a nucleophile due to the presence of lone pairs on the oxygen atom.

### Step-by-Step Solution:

1. **Analyze BF₃ (Boron Trifluoride)**:
- BF₃ is an electron-deficient molecule because boron has only six electrons in its valence shell and can accept a pair of electrons.
- When water (a nucleophile) approaches BF₃, it can donate a lone pair to boron, leading to hydrolysis.
- **Conclusion**: BF₃ undergoes hydrolysis.

2. **Analyze SiCl₄ (Silicon Tetrachloride)**:
- SiCl₄ has a tetrahedral geometry and silicon can expand its octet by utilizing its vacant d-orbitals.
- When water approaches SiCl₄, it can also react and undergo hydrolysis, forming silicic acid (H₄SiO₄).
- **Conclusion**: SiCl₄ undergoes hydrolysis.

3. **Analyze PCl₅ (Phosphorus Pentachloride)**:
- PCl₅ has a trigonal bipyramidal structure and phosphorus can utilize its vacant d-orbitals to accept electrons.
- PCl₅ can react with water, leading to the formation of phosphoric acid (H₃PO₄) and hydrochloric acid (HCl).
- **Conclusion**: PCl₅ undergoes hydrolysis.

4. **Analyze SF₆ (Sulfur Hexafluoride)**:
- SF₆ has an octahedral geometry and sulfur has a full octet with no available d-orbitals for bonding.
- The presence of six fluorine atoms creates significant steric hindrance, preventing water from approaching and reacting with the sulfur atom.
- **Conclusion**: SF₆ does not undergo hydrolysis.

### Final Answer:
Among the given compounds, only **SF₆** is inert to hydrolysis. Therefore, the number of halides that are inert to hydrolysis is **1**.