Identify conjugate acids, bases and pairs
Track a proton transfer to name a conjugate acid and base and identify the two pairs in a reaction.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 25. Use the equilibrium expressions and data stated in the question; written practice is self-checked and human subject-teacher review remains pending.
Before you begin
- Use AS equilibrium expressions, mole calculations, logarithms, and Brønsted–Lowry acid–base ideas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A base accepts H⁺. What does it form?
What you will learn
- Use conjugate acid and conjugate base consistently
- Identify conjugate pairs in proton-transfer equations
- Relate each pair by the gain or loss of one H⁺
A conjugate pair differs by one proton
A Brønsted–Lowry acid donates H⁺; a Brønsted–Lowry base accepts H⁺. When an acid donates H⁺, the species left is its conjugate base. When a base accepts H⁺, the product is its conjugate acid.
In a proton-transfer equation, identify the species before and after each H⁺ transfer. The acid and its conjugate base have formulas differing by H⁺; the base and its conjugate acid do too. Charges must change by one unit when the proton is gained or lost.
Label both pairs, not just the acid
For CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺, ethanoic acid donates H⁺ to water. CH₃COOH/CH₃COO⁻ is one conjugate pair, and H₂O/H₃O⁺ is the other.
Water can act as an acid or a base depending on its reaction partner. Do not decide its role from the formula alone; follow the proton transfer shown.
Pause and explain
In HA + H₂O ⇌ A⁻ + H₃O⁺, which species form a conjugate pair?
Worked example
For NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, identify the acid, base and conjugate pairs.
Show the worked solution
- NH₃ gains H⁺ to become NH₄⁺.
- H₂O loses H⁺ to become OH⁻.
- Pair each reactant with the product differing by one H⁺.
Answer H₂O is the acid and OH⁻ its conjugate base; NH₃ is the base and NH₄⁺ its conjugate acid. The pairs are H₂O/OH⁻ and NH₄⁺/NH₃.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
For HCl + H₂O → H₃O⁺ + Cl⁻, label the acid, base and both conjugate pairs.
Give me a hint
Follow the H⁺ from HCl to water.
Compare my reasoning
- HCl donates H⁺.
- H₂O accepts H⁺.
- Match each reactant to the product differing by one H⁺.
HCl is the acid and Cl⁻ its conjugate base; H₂O is the base and H₃O⁺ its conjugate acid. The pairs are HCl/Cl⁻ and H₃O⁺/H₂O.
Look for these in your work
- Donor and acceptor identified
- Both conjugate products matched
- Pairs differ by one proton
For HSO₄⁻ + H₂O ⇌ SO₄²⁻ + H₃O⁺, identify the two conjugate pairs.
Give me a hint
Compare the formulas and charges before and after proton transfer.
Compare my reasoning
- HSO₄⁻ loses H⁺ to become SO₄²⁻.
- H₂O gains H⁺ to become H₃O⁺.
- Write each acid/conjugate-base pair.
The pairs are HSO₄⁻/SO₄²⁻ and H₃O⁺/H₂O. HSO₄⁻ donates the proton; water accepts it.
Look for these in your work
- Charge change checked
- Both pairs identified
- Proton transfer direction stated
A learner labels NH₄⁺ as the base in NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. Correct the label and explain the pair.
Give me a hint
Ask which species gains H⁺ and which loses it.
Compare my reasoning
- NH₃ gains H⁺ to become NH₄⁺.
- H₂O donates H⁺ to become OH⁻.
- Name each reactant's role and conjugate product.
NH₃ is the base because it accepts H⁺; NH₄⁺ is its conjugate acid. H₂O is the acid and OH⁻ its conjugate base.
Look for these in your work
- Base identified as proton acceptor
- Conjugate acid named
- Water pair included
Common mistakes
- Pairing species that differ by more or less than one proton
- Calling the proton acceptor an acid
- Ignoring the charge change when H⁺ is gained or lost
What can you explain now?
A base accepts H⁺. What does it form?
Compare with the explanation
Its conjugate acid
The proton-accepting species gains H⁺ to become its conjugate acid.
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Make it stick
How can gaining or losing one H⁺ identify both members of a conjugate acid–base pair?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.