Strategy for comparing acidity:

1. Write the conjugate base for each acid
2. Compare stability of conjugate bases
3. More stable conjugate base = stronger acid

Factors stabilizing conjugate base (in priority order):

1. Resonance (if applicable — carboxylate vs alkoxide)
2. Electronegativity of atom bearing charge
3. Size of atom (for same group elements: HI > HBr > HCl > HF)
4. Inductive effects of substituents
5. s-character of orbital bearing lone pair

Common comparison series:

• Phenol (~10) > H2O (~15.7) > Ethanol (~16) > Acetylene (~25) > NH3 (~38) > Ethane (~50) — pKa values
• Ortho effect: o-substituted benzoic acids are more acidic than para (steric inhibition of resonance + intramolecular H-bonding in conjugate base)