How to Identify Conjugate Acid–Base Pairs

Conjugate pair questions look simple, but they catch out a surprising number of students because of one small slip: forgetting that adding or removing a proton changes the charge. This guide gives you a method that works every time, then lots of practice so the pattern becomes automatic.

If the idea of proton donors and acceptors is new to you, read Brønsted–Lowry acids and bases first.

The core rule

A conjugate acid–base pair is two species that differ by exactly one proton (H⁺).

• The member with the extra H⁺ is the conjugate acid.
• The member without it is the conjugate base.

Because H⁺ carries a +1 charge, adding it raises the charge by one and removing it lowers the charge by one. Every conjugate pair therefore differs in two ways: one H, and one unit of charge.

Conjugate acid	Conjugate base	Check
HCl	Cl⁻	lose H, charge 0 → −1
H₃O⁺	H₂O	lose H, charge +1 → 0
H₂O	OH⁻	lose H, charge 0 → −1
NH₄⁺	NH₃	lose H, charge +1 → 0
HSO₄⁻	SO₄²⁻	lose H, charge −1 → −2

Part 1: writing the conjugate of a single species

To write the conjugate base: remove one H and subtract one from the charge.

• HNO₂ → NO₂⁻
• H₂S → HS⁻
• HS⁻ → S²⁻
• NH₃ → NH₂⁻ (the amide ion; ammonia is a very, very weak acid)

To write the conjugate acid: add one H and add one to the charge.

• F⁻ → HF
• CO₃²⁻ → HCO₃⁻
• CH₃NH₂ → CH₃NH₃⁺
• H₂O → H₃O⁺

Which H to remove? For oxyacids and carboxylic acids, it’s the hydrogen bonded to oxygen. In ethanoic acid, CH₃COOH, only the –COOH hydrogen is acidic, so the conjugate base is CH₃COO⁻, not CH₂COOH⁻. When writing formulas, chemists usually keep the acidic hydrogen at the front (HNO₃) or the end (CH₃COOH) so it’s easy to spot.

Part 2: finding both pairs in an equation

Every Brønsted–Lowry reaction contains two conjugate pairs. Use this four-step method.

Step 1. Compare each reactant with each product. Look for species that differ by exactly one H and one unit of charge.

Step 2. Link each reactant to its partner on the other side. A conjugate pair always has one member among the reactants and one among the products. Two species on the same side can never be a pair.

Step 3. Decide which reactant lost the proton. That reactant is the acid; its partner in the products is the conjugate base.

Step 4. The other reactant gained the proton. It’s the base; its product partner is the conjugate acid.

Worked example 1

HCN + H₂O ⇌ CN⁻ + H₃O⁺

• HCN → CN⁻: lost H⁺. HCN is the acid, CN⁻ is its conjugate base.
• H₂O → H₃O⁺: gained H⁺. H₂O is the base, H₃O⁺ is its conjugate acid.

Worked example 2

NH₃ + HCO₃⁻ ⇌ NH₄⁺ + CO₃²⁻

• NH₃ → NH₄⁺: gained H⁺. Base / conjugate acid.
• HCO₃⁻ → CO₃²⁻: lost H⁺. Acid / conjugate base.

Notice that HCO₃⁻, often thought of as a base, acts as an acid here because ammonia is a stronger base than carbonate.

Worked example 3

H₂PO₄⁻ + OH⁻ → HPO₄²⁻ + H₂O

• H₂PO₄⁻ → HPO₄²⁻: lost H⁺. Acid / conjugate base.
• OH⁻ → H₂O: gained H⁺. Base / conjugate acid.

Worked example 4: a trap

HCl + NH₃ → NH₄⁺ + Cl⁻

Someone might pair HCl with NH₄⁺ because both “look acidic”. But HCl and NH₄⁺ don’t differ by one H: they have completely different atoms. Always check the formula, not the vibe. The correct pairs are HCl/Cl⁻ and NH₄⁺/NH₃.

Part 3: polyprotic ladders

Acids that can lose more than one proton form a ladder of conjugates. Each rung is a conjugate pair with the rung below it.

Phosphoric acid:

H₃PO₄ ⇌ H₂PO₄⁻ ⇌ HPO₄²⁻ ⇌ PO₄³⁻

Carbonic acid:

H₂CO₃ ⇌ HCO₃⁻ ⇌ CO₃²⁻

Sulfuric acid:

H₂SO₄ → HSO₄⁻ ⇌ SO₄²⁻

The species in the middle of each ladder (H₂PO₄⁻, HPO₄²⁻, HCO₃⁻, HSO₄⁻) are amphiprotic: each is the conjugate base of the rung above and the conjugate acid of the rung below. That dual role is exactly what makes phosphate and hydrogencarbonate such good buffers. See buffers explained.

A common error: H₃PO₄ and HPO₄²⁻ are not a conjugate pair. They differ by two protons. Conjugates are always neighbours on the ladder.

Part 4: using conjugate pairs to predict direction

The strength of an acid and its conjugate base are inversely linked. A strong acid has a very weak conjugate base; a weak acid has a moderately strong one.

In any proton-transfer equilibrium, the position lies on the side of the weaker acid and weaker base. So if you know which of the two acids in an equation is weaker, you know which way the equilibrium favours.

In example 2 above, HCO₃⁻ (pKa ≈ 10.3) is a slightly weaker acid than NH₄⁺ (pKa ≈ 9.25). The equilibrium therefore lies a little to the left, on the side of HCO₃⁻ and NH₃. Both acids are weak and close in strength, so neither side dominates strongly.

Why conjugate pairs matter beyond the exam

Conjugate pairs aren’t just a labelling exercise. Two of the most useful tools in chemistry are built from them.

Buffers are mixtures of a weak acid and its conjugate base in similar amounts, such as ethanoic acid with sodium ethanoate. Added acid is soaked up by the conjugate base; added alkali is soaked up by the weak acid. The pH barely moves.

Indicators are weak acids whose conjugate base has a different colour. In methyl orange, the acid form is red and the conjugate base is yellow. The colour you see tells you which member of the pair dominates, and that depends on the pH of the solution.

In both cases, if you can pick out the conjugate pair, you can explain how the system works.

Practice questions

A. Write the conjugate base of:

1. HBr 2. H₂SO₃ 3. HPO₄²⁻ 4. H₂O 5. NH₄⁺

B. Write the conjugate acid of: 6. NO₃⁻ 7. HS⁻ 8. C₆H₅NH₂ 9. PO₄³⁻ 10. H₂O

C. Identify the acid, base, conjugate acid and conjugate base: 11. HNO₃ + H₂O → NO₃⁻ + H₃O⁺ 12. CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻ 13. HSO₄⁻ + HCO₃⁻ ⇌ SO₄²⁻ + H₂CO₃ 14. NH₂⁻ + H₂O → NH₃ + OH⁻ 15. H₂O + H₂O ⇌ H₃O⁺ + OH⁻

Answers

1. Br⁻ 2. HSO₃⁻ 3. PO₄³⁻ 4. OH⁻ 5. NH₃
2. HNO₃ 7. H₂S 8. C₆H₅NH₃⁺ 9. HPO₄²⁻ 10. H₃O⁺
3. Acid HNO₃, base H₂O, conjugate base NO₃⁻, conjugate acid H₃O⁺.
4. Base CH₃COO⁻, acid H₂O, conjugate acid CH₃COOH, conjugate base OH⁻.
5. Acid HSO₄⁻, base HCO₃⁻, conjugate base SO₄²⁻, conjugate acid H₂CO₃.
6. Base NH₂⁻, acid H₂O, conjugate acid NH₃, conjugate base OH⁻.
7. One H₂O is the acid (conjugate base OH⁻); the other is the base (conjugate acid H₃O⁺).

Mistakes to avoid

• Forgetting the charge change. NH₃’s conjugate acid is NH₄⁺, never NH₄.
• Pairing species on the same side. Pairs always cross the arrow.
• Skipping a rung on a polyprotic ladder. Conjugates differ by one proton only.
• Assuming a species is always an acid or always a base. HCO₃⁻, H₂O and HPO₄²⁻ can play either role.

Key takeaways

• Conjugate pairs differ by exactly one H⁺: one hydrogen and one unit of charge.
• Each acid–base equation contains two pairs, one crossing the arrow in each direction.
• Polyprotic acids form ladders of conjugates; the middle species are amphiprotic.
• Equilibrium favours the side with the weaker acid and base.
• For the wider theory, see acid–base definitions and strong vs weak acids.