A proton cannot leave one place without arriving somewhere else. That single fact means acids and bases always appear in twos — and once you can spot the pairs, half the questions in this topic answer themselves.
📚 What you need to know
When an acid donates its proton, what remains is its conjugate base.
When a base accepts a proton, what forms is its conjugate acid.
A conjugate pair differs by exactly one H+ — nothing else.
The charge of a pair differs by exactly one unit.
Every proton transfer equation contains two conjugate pairs.
A strong acid has a weak conjugate base, and a weak acid has a stronger one.
One transfer, two pairs
Take ethanoic acid in water. The acid hands a proton to the water, so on the right-hand side you are left with the acid minus a proton, and the water plus a proton. Those two changes are the two pairs.
The pairs cross over the equilibrium arrow. Reactant acid pairs with product base; reactant base pairs with product acid. Two species on the same side are never a pair.
“Conjugate” simply means related. There is nothing deep hiding in the word — CH3COOH and CH3COO– are related because one is the other with a proton removed. If you can find two formulas that differ by a single H and a single charge, you have found a pair.
Finding the partner
The bookkeeping is the whole skill. Remove an H+ and you must also remove one positive charge, which is the same as adding one negative. It sounds obvious written down; it is where most of the lost marks are.
Note the third row. A positive ion loses a proton and becomes neutral — the charge still drops by one, it just does not end up negative.
🧩 Writing a conjugate partner
Decide whether you need the conjugate acid (add H+) or the conjugate base (remove H+).
Change the number of hydrogen atoms by one. Nothing else in the formula changes.
Adjust the charge by one unit, in the same direction as the proton.
Check it is sensible: you cannot remove a hydrogen from a species that has none.
Strong acid, weak conjugate base
Now bring in what you know about equilibrium. Hydrochloric acid dissociates so completely that the reverse reaction is effectively invisible. That reverse reaction is Cl– grabbing a proton back — so if it never happens, Cl– must be a hopeless proton acceptor.
Ethanoic acid is the other case. It only partly dissociates, which means plenty of ethanoate ions are pulling protons back off water. CH3COO– is a far better base than Cl– ever is.
The general rule
the stronger the acid, the weaker its conjugate base the stronger the base, the weaker its conjugate acid
Acid
Strength as an acid
Conjugate base
Strength as a base
HCl
strong
Cl–
extremely weak
H3O+
strong
H2O
weak
CH3COOH
weak
CH3COO–
moderate
H2O
extremely weak
OH–
strong
This is the reason a solution of sodium ethanoate is alkaline while a solution of sodium chloride is neutral. Ethanoate ions are basic enough to take protons from water and release OH–; chloride ions simply sit there.
WORKED EXAMPLE
Label all four species in each equilibrium as acid, base, conjugate acid or conjugate base. (a) HF(aq) + H2O(l) ⇌ F–(aq) + H3O+(aq) (b) NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH–(aq)
(a) which one lost a hydrogen?HF became F⁻, so HF is the acid and F⁻ is its conjugate base. H₂O became H₃O⁺, so water is the base and H₃O⁺ is its conjugate acid.HF acid · H₂O base · F⁻ conj. base · H₃O⁺ conj. acid(b) same methodNH₃ gained a hydrogen, so it is the base. Water lost one, so this time water is the acid.NH₃ base · H₂O acid · NH₄⁺ conj. acid · OH⁻ conj. baseThe pairs are diagonal in both cases. If you have paired two species on the same side of the arrow, something has gone wrong.
WORKED EXAMPLE
Give the conjugate base of: H2PO4–, HNO3, H2S. Give the conjugate acid of: OH–, CO32–, NH3.
Conjugate bases — remove one H⁺H₂PO₄⁻ → HPO₄²⁻HNO₃ → NO₃⁻H₂S → HS⁻Conjugate acids — add one H⁺OH⁻ → H₂OCO₃²⁻ → HCO₃⁻NH₃ → NH₄⁺Every answer changes the hydrogen count by one and the charge by one. Check both before you move on.
WORKED EXAMPLE
A student writes that in the equilibrium HCOOH(aq) + H2O(l) ⇌ HCOO–(aq) + H3O+(aq), the species HCOOH and H3O+ are a conjugate pair. Explain the error and give the correct pairs.
Test the claimHCOOH and H₃O⁺ are completely different substances. A pair must be the same species with and without one proton.they are not related by a single H⁺The correct pairsHCOOH / HCOO⁻H₃O⁺ / H₂OMethanoic acid is a weak acid, so its conjugate base HCOO⁻ is a reasonably good proton acceptor.
💡 Exam tip
Look for the species that differ by one H. That is the entire test.
Pairs are always diagonal across the arrow, never side by side.
Adjust the charge as well as the formula. Losing H+ from a 1– ion gives a 2– ion.
If a question asks for the conjugate base of a species with no hydrogen, check you have read it correctly — it cannot have one.
Learn the phrase “strong acids have weak conjugate bases”; it is worth a mark on its own.
⚠️ Common mix-up
Pairing two reactants or two products. The two members of a pair are always on opposite sides.
Changing the formula by more than one H, e.g. giving SO42– as the conjugate base of H2SO4.
Forgetting the charge, writing CO32– → HCO32–.
Thinking a strong acid gives a strong conjugate base. It is the opposite, every time.
Assuming water is always the base. In the ammonia equilibrium it is the acid.
Up next: Amphiprotic Species — you have now seen water behave as an acid on one page and a base on another. That is not a contradiction; it has a name.
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