Which molecule has the highest boiling point ?

Options

A. HI
B. HF
C. HBr
D. HCI

Answer

To determine which molecule has the highest boiling point among HI, HF, HBr, and HCl, we will analyze the factors that influence boiling points, such as molecular mass and intermolecular forces.

### Step-by-Step Solution:

1. **Identify the Molecules**: The molecules in question are HI (hydrogen iodide), HF (hydrogen fluoride), HBr (hydrogen bromide), and HCl (hydrogen chloride).

2. **Consider Molecular Mass**: Generally, boiling points increase with molecular mass because larger molecules have greater van der Waals forces due to their size.
- Molecular masses are approximately:
- HI: 127 g/mol
- HBr: 81 g/mol
- HCl: 36.5 g/mol
- HF: 20 g/mol

3. **Analyze Intermolecular Forces**:
- HF exhibits hydrogen bonding due to the highly electronegative fluorine atom, which creates a strong attraction between HF molecules.
- The other hydrogen halides (HI, HBr, HCl) primarily exhibit van der Waals forces, which are weaker than hydrogen bonds.

4. **Rank the Molecules by Boiling Point**:
- HF has strong hydrogen bonding, which significantly increases its boiling point despite its lower molecular mass.
- HI, being the heaviest, has a high boiling point due to its size and van der Waals forces, but it does not have hydrogen bonding.
- HBr and HCl have lower boiling points than both HF and HI due to their smaller sizes and weaker intermolecular forces.

5. **Conclusion**:
- HF has the highest boiling point due to the presence of hydrogen bonding, which outweighs the effects of molecular mass in this case.
- Therefore, the answer is **HF (hydrogen fluoride)**.