Which of the following Bronsted acids has the wekest conjugate base ?

Options

A. `H_(2)O`
B. `HCN`
C. `HCOOH`
D. `HF`

Explanation

To determine which Bronsted acid has the weakest conjugate base, we need to follow these steps:

### Step 1: Identify the Bronsted acids and their conjugate bases.
- **Water (H2O)** → Conjugate base: OH⁻
- **Hydrogen cyanide (HCN)** → Conjugate base: CN⁻
- **Formic acid (HCOOH)** → Conjugate base: HCOO⁻
- **Hydrofluoric acid (HF)** → Conjugate base: F⁻

### Step 2: Determine the strength of the acids.
The strength of the acids can be ranked based on their ability to donate protons (H⁺):
1. **HF** (strongest acid)
2. **HCOOH** (formic acid)
3. **HCN** (hydrogen cyanide)
4. **H2O** (weakest acid)

### Step 3: Relate the strength of the acids to the strength of their conjugate bases.
The strength of a conjugate base is inversely related to the strength of its corresponding acid. This means:
- The stronger the acid, the weaker its conjugate base.

### Step 4: Identify the weakest conjugate base.
Since HF is the strongest acid among the options, its conjugate base (F⁻) will be the weakest.

### Conclusion:
Thus, the Bronsted acid with the weakest conjugate base is **HF**.

---