MCQ Detailed View

The dissociation constant (Ka) is an important measure of the strength of an acid. It shows how easily an acid ionizes in water to release hydrogen ions (H⁺). A higher Ka value means a stronger acid, while a lower Ka value indicates a weaker acid.

The given Ka value is 1.3 × 10⁻¹⁰, which represents a very weak acid. To determine the correct compound, we need to compare the known Ka values of the given options.

Acetic acid (CH₃COOH) is a weak acid, but it is much stronger than phenol. Its Ka value is about 1.8 × 10⁻⁵, which is five orders of magnitude larger than 1.3 × 10⁻¹⁰. Therefore, acetic acid cannot be the correct answer.

Alcohols (R–OH) are extremely weak acids. Their Ka values are around 10⁻¹⁶ to 10⁻¹⁸, which is much smaller than the given value. This means alcohols are weaker acids than phenol, and they do not match the Ka value provided.

Water (H₂O) is considered neutral, and instead of Ka, it has an ionic product constant (Kw = 1 × 10⁻¹⁴ at 25°C). Therefore, water is not the correct choice.

Phenol (C₆H₅OH) is a weak acid, but it is significantly more acidic than alcohols due to resonance stabilization of its conjugate base, the phenoxide ion. The experimental Ka value of phenol is approximately 1.3 × 10⁻¹⁰, which perfectly matches the given data.

This shows that the Ka value of 1.3 × 10⁻¹⁰ belongs to phenol. The acidic nature of phenol arises because, after losing a proton, the negative charge on oxygen is delocalized into the aromatic ring through resonance. This stabilization makes phenol acidic, but still weaker than carboxylic acids like acetic acid.

Thus, the dissociation constant given corresponds to phenol, confirming its position as a weak organic acid stronger than alcohols but weaker than carboxylic acids.