3.1.2—Conjugate acid-base pairs
- Syllabus
- First assessment 2025
- Objective
- 3.1.2
- Level
- HL
Conjugate Acid–Base Pairs
A conjugate base is what remains after an acid donates one proton. A conjugate acid is formed when a base accepts one proton; the pair differs by exactly one H+.
Remove H+ to find the conjugate base or add H+ to find the conjugate acid, then check the charge changes by one unit.
NH₄⁺/NH₃ and H₂CO₃/HCO₃⁻ are conjugate pairs because each pair differs by one H⁺. Removing H⁺ lowers charge by one; adding H⁺ raises it by one. Do not pair species merely because they occur on opposite sides of an equation—trace the specific proton transfer.
Deducing Conjugate Formulae
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
A solution of nitrous acid contains two conjugate acid-base pairs.
State the formulas of the conjugate acid and conjugate base in each pair.
Conjugate acid:
Conjugate base:
Conjugate acid:
Conjugate base:
conjugate acid H3O+«(aq)» AND conjugate base H2O<(l)»
conjugate acid HNO2 «(aq)» AND conjugate base NO2−«(aq)»
Proton Transfer Reactions Summary
Retrieve the route: track proton transfer and conjugates, calculate pH and Kw, distinguish strength, balance neutralization, read titration curves, use Ka/Kb and hydrolysis, select indicators, and explain and calculate buffer behaviour.
Check donor versus acceptor, one-proton differences, logarithm direction, ion comparison, strength versus concentration, equivalence versus endpoint, pKa landmarks, conjugate equations and dilution ratios.