Choose the correct one which is the weakest acid?

Options

A. HCl
B. HF
C. `H_2SO_4`
D. `HNO_3`

Answer

To determine which of the given acids is the weakest, we will analyze the strength of each acid based on their dissociation in water and the stability of the conjugate bases formed after dissociation.

### Step-by-Step Solution:

1. **Identify the Acids**: The acids given in the question are HCl (hydrochloric acid), HF (hydrofluoric acid), H2SO4 (sulfuric acid), and HNO3 (nitric acid).

2. **Classify the Acids**:
- HCl, H2SO4, and HNO3 are classified as strong acids. Strong acids are those that completely dissociate in water to produce H⁺ ions.
- HF is classified as a weak acid because it does not completely dissociate in water.

3. **Dissociation of Acids**:
- **HCl** dissociates completely:
\[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \]
- **H2SO4** dissociates completely in two steps:
\[ \text{H2SO4} \rightarrow \text{H}^+ + \text{HSO4}^- \]
\[ \text{HSO4}^- \rightarrow \text{H}^+ + \text{SO4}^{2-} \]
- **HNO3** dissociates completely:
\[ \text{HNO3} \rightarrow \text{H}^+ + \text{NO3}^- \]
- **HF** dissociates partially:
\[ \text{HF} \rightleftharpoons \text{H}^+ + \text{F}^- \]

4. **Stability of Conjugate Bases**:
- The conjugate base of HCl is Cl⁻, which is stable due to its larger size, allowing the negative charge to be spread out.
- The conjugate base of HNO3 is NO3⁻, which is stabilized by resonance.
- The conjugate base of H2SO4 is HSO4⁻ and SO4²⁻, both of which are also stabilized by resonance.
- The conjugate base of HF is F⁻, which is less stable compared to Cl⁻ because the negative charge is concentrated on a smaller atom (fluorine).

5. **Conclusion**: Since HF does not dissociate completely and its conjugate base (F⁻) is less stable compared to the conjugate bases of the other acids, HF is the weakest acid among the four options.

### Final Answer:
The weakest acid among the given options is **HF (hydrofluoric acid)**.

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