Which of the following has lowest boiling point?

Options

A. `HF`
B. `HCl`
C. `Hl`
D. `HBr`

Explanation

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

### Step-by-Step Solution:

1. **Identify the Compounds and Their Molecular Weights:**
- HF (Hydrogen fluoride): H = 1, F = 19 → Molecular weight = 20 g/mol
- HCl (Hydrogen chloride): H = 1, Cl = 35.5 → Molecular weight = 36.5 g/mol
- HBr (Hydrogen bromide): H = 1, Br = 80 → Molecular weight = 81 g/mol
- HI (Hydrogen iodide): H = 1, I = 127 → Molecular weight = 128 g/mol

2. **Arrange the Compounds by Molecular Weight:**
- The order of molecular weights from lowest to highest is:
- HF (20 g/mol)
- HCl (36.5 g/mol)
- HBr (81 g/mol)
- HI (128 g/mol)

3. **Consider the Intermolecular Forces:**
- HF can form hydrogen bonds due to the presence of highly electronegative fluorine. This leads to a higher boiling point.
- HCl, HBr, and HI primarily exhibit dipole-dipole interactions and London dispersion forces, which are generally weaker than hydrogen bonds.

4. **Determine the Boiling Point Order:**
- Based on molecular weight alone, we would expect the boiling points to increase with molecular weight. However, due to the strong hydrogen bonding in HF, its boiling point is higher than expected.
- The expected order of boiling points based on molecular weight and intermolecular forces is:
- HF > HCl > HBr > HI

5. **Identify the Compound with the Lowest Boiling Point:**
- From the analysis, HCl has a higher boiling point than HF due to hydrogen bonding, while HBr and HI have lower boiling points due to their larger molecular weights but weaker intermolecular forces.
- Therefore, the compound with the lowest boiling point among the given options is **HI**.

### Conclusion:
The compound with the lowest boiling point is **HI**.