Weakest acid among the following is

Options

A. acetic acid
B. phenol
C. water
D. acetylene

Answer

To determine the weakest acid among the given options (acetic acid, phenol, water, and acetylene), we need to analyze the acidity of each compound based on the stability of their conjugate bases after they lose a proton (H⁺).

### Step-by-Step Solution:

1. **Identify the Compounds**:
- Acetic acid (CH₃COOH)
- Phenol (C₆H₅OH)
- Water (H₂O)
- Acetylene (C₂H₂ or CH≡CH)

2. **Understand Acidity**:
- Acids donate protons (H⁺) in aqueous solution.
- The strength of an acid is often determined by the stability of its conjugate base after it loses H⁺.

3. **Determine Conjugate Bases**:
- **Acetic Acid**: When acetic acid loses H⁺, it forms acetate ion (CH₃COO⁻).
- **Phenol**: When phenol loses H⁺, it forms phenoxide ion (C₆H₅O⁻).
- **Water**: When water loses H⁺, it forms hydroxide ion (OH⁻).
- **Acetylene**: When acetylene loses H⁺, it forms acetylide ion (C₂H⁻).

4. **Analyze Stability of Conjugate Bases**:
- **Acetate Ion (CH₃COO⁻)**: The negative charge is delocalized due to resonance between the two oxygen atoms, making it quite stable.
- **Phenoxide Ion (C₆H₅O⁻)**: The negative charge is also stabilized by resonance with the benzene ring, making it stable as well.
- **Hydroxide Ion (OH⁻)**: The negative charge on oxygen is stable due to oxygen's high electronegativity.
- **Acetylide Ion (C₂H⁻)**: The negative charge is localized on a carbon atom, which is less electronegative than oxygen. This makes the acetylide ion less stable compared to the others.

5. **Conclusion**:
- Since the acetylide ion (C₂H⁻) is the least stable due to the negative charge being on a less electronegative atom (carbon), acetylene (C₂H₂) is the weakest acid among the given options.

### Final Answer:
The weakest acid among the following is **Acetylene (C₂H₂)**.

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