A base will deprotonate any acid whose pKa is lower than that of its own conjugate acid. That one comparison answers most acid-base exam questions.
| Acid | Conjugate base | pKa | Why it matters |
|---|---|---|---|
| HI | I− | −10 | Strong acid — fully dissociated |
| HBr | Br− | −9 | Strong acid |
| H2SO4 | HSO4− | −9 | First proton |
| HCl | Cl− | −7 | Strong acid |
| H3O+ | H2O | −1.7 | Levelling acid in water |
| HNO3 | NO3− | −1.4 | Strong acid |
| HF | F− | 3.2 | Weak despite the strong bond |
| CH3COOH | CH3COO− | 4.76 | Reference carboxylic acid |
| H2CO3 | HCO3− | 6.4 | Blood buffer |
| H2S | HS− | 7.0 | More acidic than water |
| NH4+ | NH3 | 9.2 | Ammonium |
| HCN | CN− | 9.2 | Cyanide is a good nucleophile |
| C6H5OH | C6H5O− | 10.0 | Phenol — resonance-stabilised anion |
| CH3SH | CH3S− | 10.3 | Thiol beats alcohol |
| H2O | HO− | 15.7 | The one to memorise |
| CH3CH2OH | CH3CH2O− | 16 | Alcohols ≈ water |
| CH3COCH3 | Enolate | 19.2 | α-hydrogen of a ketone |
| HC≡CH | HC≡C− | 25 | Terminal alkyne — NaNH2 deprotonates it |
| NH3 | NH2− | 38 | Sodium amide is a strong base |
| CH2=CH2 | CH2=CH− | 44 | sp2 C–H |
| CH4 | CH3− | 50 | Effectively not acidic |
Lower pKa means the stronger acid. A base will deprotonate any acid whose pKa is lower than that of its own conjugate acid — that single comparison decides most acid–base questions on an exam.