acetic acid

Meaning: Acetic acid is CH₃COOH, a carboxylic acid with two carbons. In water it ionizes partly to H₃O⁺ and acetate, CH₃COO⁻. The carboxyl O–H hydrogen is the one that ionizes, because acetate spreads its negative charge over two oxygens by resonance. Pure acetic acid forms hydrogen-bonded dimers, so its boiling point (about 118 °C) is high for its size, and it mixes well with water.

Equation: Ka = [H₃O⁺][CH₃COO⁻] / [CH₃COOH], with concentrations in mol/L. For acetic acid Ka is about 1.8 × 10⁻⁵ at 25 °C, so pKa is about 4.76.

On the MCAT: A common passage titrates acetic acid with NaOH. The buffer region lies near pH = pKa. At the half-equivalence point [CH₃COOH] = [CH₃COO⁻], so pH = pKa. At the equivalence point the pH is above 7, because acetate is a weak base. You may be asked to rank acidity: electron-withdrawing groups, as in chloroacetic acid, stabilize the conjugate base and give a lower pKa. Acetic acid also reacts with alcohols to make esters (Fischer esterification) and can be converted to acetyl chloride or acetic anhydride.

Common confusion: Weak does not mean dilute. A 1 M acetic acid solution has a pH far above 0, because only a small fraction ionizes. For a weak acid, [H⁺] ≈ √(Ka × C), not C. The methyl C–H hydrogens are not acidic in water.