Which of the following compounds has the lowest boiling point ?

Options

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

Explanation

To determine which of the given compounds has the lowest boiling point, we will analyze the compounds based on their intermolecular forces and molecular weights.

### Step-by-Step Solution:

1. **Identify the Compounds**:
The compounds we are considering are HF (hydrogen fluoride), HI (hydrogen iodide), HBr (hydrogen bromide), and HCl (hydrogen chloride).

2. **Understand Intermolecular Forces**:
- HF exhibits strong hydrogen bonding due to the presence of highly electronegative fluorine. This significantly increases its boiling point.
- HCl, HBr, and HI primarily exhibit Van der Waals forces (dispersion forces), which are generally weaker than hydrogen bonds.

3. **Compare Molecular Weights**:
- The molecular weights of the compounds are as follows:
- HF: 20 g/mol
- HCl: 36.5 g/mol
- HBr: 80 g/mol
- HI: 127 g/mol
- Generally, higher molecular weight compounds have higher boiling points due to increased Van der Waals forces.

4. **Analyze Boiling Points**:
- HF has the highest boiling point due to hydrogen bonding.
- Among HCl, HBr, and HI, the boiling points will increase with increasing molecular weight:
- HI > HBr > HCl.
- Therefore, HCl will have a lower boiling point than HBr and HI.

5. **Determine the Lowest Boiling Point**:
- Since HF has the highest boiling point and HCl has a lower boiling point than HBr and HI, we need to compare HCl with the other gaseous compounds.
- HCl has a lower boiling point than both HBr and HI, but we need to compare it with itself.
- HCl has the lowest boiling point among the gaseous compounds listed.

6. **Conclusion**:
- Among HF, HCl, HBr, and HI, the compound with the lowest boiling point is HCl.

### Final Answer:
The compound with the lowest boiling point is **HCl**.