Which of the following halogen hydrides will have the weakest conjugate base?

Options

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

Answer

To determine which halogen hydride has the weakest conjugate base, we need to analyze the acidity of the halogen hydrides: HF, HCl, HBr, and HI.

### Step-by-Step Solution:

1. **Identify the Halogen Hydrides**: The halogen hydrides in question are HF (hydrogen fluoride), HCl (hydrogen chloride), HBr (hydrogen bromide), and HI (hydrogen iodide).

2. **Understand Acidity and Conjugate Bases**: The strength of an acid is related to its ability to donate protons (H⁺ ions). When an acid donates a proton, it forms its conjugate base. The stronger the acid, the weaker its conjugate base.

3. **Electronegativity Trend**: The electronegativity of the halogens decreases as we move down the group in the periodic table:
- Fluorine (F) > Chlorine (Cl) > Bromine (Br) > Iodine (I)

4. **Acidity of Halogen Hydrides**:
- HF is a weak acid because fluorine is highly electronegative and holds onto the hydrogen tightly.
- HCl is a stronger acid than HF.
- HBr is stronger than HCl.
- HI is the strongest acid among them because iodine is the least electronegative and can easily lose the H⁺ ion.

5. **Determine the Order of Acidity**: The order of acidity from strongest to weakest is:
- HI > HBr > HCl > HF

6. **Identify the Weakest Conjugate Base**: Since HI is the strongest acid, its conjugate base (I⁻) will be the weakest. Therefore, the weakest conjugate base corresponds to the strongest acid.

### Conclusion:
The halogen hydride that has the weakest conjugate base is **HI**.