A proton cannot vanish. If one molecule gives it away, another one is holding it. So every acid comes with a shadow — the leftover piece after the proton has gone. That shadow is its conjugate base, and once you can spot the pairs, a whole class of exam questions becomes almost free marks.
📘 What you need to know
A conjugate acid–base pair is two species that differ by exactly one H+ and nothing else.
“Conjugate” just means linked or related. It is not a special kind of chemistry.
Take a proton off a species and you get its conjugate base.
Add a proton to a species and you get its conjugate acid.
Removing H+ makes the charge go one unit more negative. Adding H+ makes it go one unit more positive.
Every proton-transfer equation contains two pairs — one on each side of the arrow.
The stronger the acid, the weaker its conjugate base, and the other way round.
Where the pairs come from
Take ethanoic acid dissolving in water. It hands a proton to a water molecule and sets up an equilibrium.
The reaction everything else is built on
CH3COOH + H2O ↔ CH3COO− + H3O+
Look at what happened to the ethanoic acid. It lost one H+ and became CH3COO−. Those two are the same molecular skeleton, one with the proton attached and one without. That is a conjugate pair.
Now look at the water. It gained one H+ and became H3O+. Same skeleton, one proton different. That is the second pair.
Because the reaction is reversible, the products can go back the other way. When they do, CH3COO− takes a proton from H3O+, so the ethanoate is behaving as a base and the H3O+ as an acid. That is exactly why we call them the conjugate base and the conjugate acid.
The pairs never sit next to each other. Each one is split by the arrow, so a pair is always one reactant plus one product.
Making a conjugate from scratch
Exam questions often skip the equation and ask you straight out: “give the conjugate base of HSO4−“. This is a two-step job with no thinking required once you know the rule.
🧩 Building a conjugate in ten seconds
Asked for a conjugate base? Delete one H from the formula.
Asked for a conjugate acid? Add one H to the formula.
Now fix the charge. Deleting H makes it one more negative; adding H makes it one more positive.
Write the charge as a superscript and check the rest of the formula is untouched. If any other atom changed, you have gone wrong.
So the conjugate base of HSO4− is SO42−: one H gone, charge from 1− to 2−. And the conjugate acid of HSO4− is H2SO4: one H added, charge from 1− to 0.
Notice HSO4− just gave you both. Plenty of species can go either way, and that is the whole of the next page.
A table worth memorising
These come up again and again. Cover the right-hand column and test yourself.
Acid
Its conjugate base
What the charge did
HCl
Cl−
0 → 1−
HNO3
NO3−
0 → 1−
CH3COOH
CH3COO−
0 → 1−
H2O
OH−
0 → 1−
H3O+
H2O
1+ → 0
NH4+
NH3
1+ → 0
H2SO4
HSO4−
0 → 1−
HCO3−
CO32−
1− → 2−
Read the table backwards too. Right column to left column is “add a proton”, so NH3 is the base and NH4+ is its conjugate acid. One table, twice the value.
Strong acid, weak partner
Here is the part that feels strange the first time and then never leaves you.
HCl is a strong acid: in water it hands its proton over almost completely and the reverse reaction barely happens. But the reverse reaction is Cl− taking a proton back. If it barely happens, then Cl− is terrible at accepting protons. In other words, Cl− is a very weak base.
Flip it round. Ethanoic acid is weak, so it holds onto its proton fairly well and the reverse reaction is easy. That means CH3COO− takes protons back happily — it is a noticeably stronger base than Cl−.
The single arrow on the left and the double arrow on the right are doing real work here. A one-way arrow means the conjugate base is not strong enough to pull the proton back.
Worked examples
WORKED EXAMPLE
Identify the two conjugate acid–base pairs in: HCOOH(aq) + CN−(aq) ↔ HCOO−(aq) + HCN(aq)
Step 1: follow the first reactant acrossHCOOH loses one H to become HCOO−. Same skeleton, one proton apart.Step 2: follow the second reactant acrossCN− gains one H to become HCN. Same skeleton, one proton apart.Step 3: name each half of each pair
HCOOH gave the proton, so it is the acid and HCOO− is its conjugate base. CN− took it, so it is the base and HCN is its conjugate acid.
Pair 1: HCOOH / HCOO− Pair 2: CN− / HCNa pair is never two reactants and never two products — always one of each
WORKED EXAMPLE
Give the conjugate acid and the conjugate base of H2PO4−.
Step 1: conjugate acid means add one HH2PO4− + H+ → H3PO4
Charge goes from 1− up to 0.
Step 2: conjugate base means take one H offH2PO4− − H+ → HPO42−
Charge goes from 1− down to 2−.
Step 3: check nothing else changed
One phosphorus and four oxygens in all three. Good.
Conjugate acid H3PO4, conjugate base HPO42−the P and the four O stay put — only the H count and the charge move
WORKED EXAMPLE
HF is a weaker acid than HBr. Which is the stronger base, F− or Br−? Explain.
Step 1: write down what “weaker acid” means
HF does not give its proton away easily, so the reverse reaction happens a lot.
Step 2: say what the reverse reaction actually isF− + H+ → HF
That is F− accepting a proton, which is exactly what a base does.
Step 3: apply the rule
Weaker acid means stronger conjugate base, so F− beats Br−.
F− is the stronger baseanswer the “why” by naming the reverse reaction, not just quoting the rule
💡 Exam tip
Always quote the pair as two species with a slash, acid first: HCOOH / HCOO−. Examiners want both halves named.
When asked to label an equation, write acid, base, conjugate base and conjugate acid under the four species so the pairing is obvious.
Check the charge every single time. It is the fastest way to catch a missing or extra hydrogen.
If a species has more than one hydrogen, you still only move one. H2SO4 goes to HSO4−, not straight to SO42−.
For “which is the stronger base” questions, find the acid strengths first and then flip them.
Metal ions such as Na+ are spectators. They are not part of any conjugate pair, so leave them out of your answer.
⚠ Common mix-up
Pairing the two reactants together. A pair is always split by the arrow: one reactant with one product.
Changing more than the hydrogen. The conjugate base of HNO3 is NO3−, not NO2−. Only the H leaves.
Forgetting the charge or getting the sign backwards. Losing a positive proton makes a species more negative, which surprises a lot of people.
Thinking a strong acid gives a strong conjugate base. It is the exact opposite, and this is tested almost every year.
Assuming “conjugate” means something exotic. It only means related by one proton.
Writing H2O as the conjugate base of H2O. A species is never its own conjugate — the partner of H2O is OH− one way and H3O+ the other.
Up next: Amphiprotic Species. You have already seen H2O, HSO4− and H2PO4− work both ways. Now we name that behaviour and separate it from a word that looks almost identical.
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