The pKa of phenol is approximately x (nearest integer).

Solution

Chemistry ma’am · 3 steps

Place phenol on the acidity ladder: stronger than alcohols, weaker than carboxylic acids.

1. Why phenol is acidic. When phenol loses H⁺, the negative charge on the phenoxide oxygen spreads into the benzene ring by resonance. This stabilises C₆H₅O⁻, so phenol gives up its proton far more easily than an alcohol does.
2. Why only weakly. In phenoxide, resonance pushes the charge onto ring carbons, which are less electronegative than oxygen. In a carboxylate, the charge is shared equally between two oxygen atoms. So phenoxide is less stable than carboxylate, and phenol is a weaker acid than a carboxylic acid.

   Compound	Approximate pKₐ	Anion stabilised by
   Ethanol, C₂H₅OH	about 16	nothing, and the alkyl group pushes electrons in
   Phenol, C₆H₅OH	about 10	resonance into the ring, with charge on carbon
   Acetic acid, CH₃COOH	about 4.8	resonance between two equal oxygen atoms

3. So. Phenol has Kₐ ≈ 1 × 10⁻¹⁰, so pKₐ = -log Kₐ ≈ 10.

The common slip: 16. Using the pKₐ of an alcohol such as ethanol gives 16. Resonance in phenoxide makes phenol about a million times more acidic than ethanol.

The idea to keep. Remember the three anchors: alcohol about 16, phenol about 10, carboxylic acid about 4 to 5. The more the anion's charge is spread out, the lower the pKₐ.

Answer from the NTA final answer key. Solution by PrepWiser.