A conjugate acid is the species formed when a Brønsted-Lowry base accepts a proton (H⁺). It is the base plus one H and one unit of positive charge, like NH₄⁺ being the conjugate acid of NH₃. Every acid-base reaction on the AP exam contains two conjugate acid-base pairs.
A conjugate acid is what a base turns into after it accepts a proton (H⁺). In Brønsted-Lowry terms, a base is a proton acceptor, and once it grabs that proton it becomes an acid capable of donating it right back. So NH₃ becomes NH₄⁺, H₂O becomes H₃O⁺, and NO₂⁻ becomes HNO₂. The recipe is simple. Take the base, add one H, and add one to the charge.
Every proton-transfer reaction has two conjugate acid-base pairs, and each pair differs by exactly one H⁺. In HNO₂ + H₂O ⇌ NO₂⁻ + H₃O⁺, the pairs are HNO₂/NO₂⁻ and H₃O⁺/H₂O. There's also an inverse strength relationship you'll use constantly. Strong bases have very weak conjugate acids, and weak bases (like ammonia) have conjugate acids strong enough to matter in equilibrium problems. That seesaw is exactly what topic 8.6 asks you to explain using molecular structure.
This term shows up in two units. In Unit 4, LO 4.8.A asks you to identify Brønsted-Lowry acids, bases, and conjugate acid-base pairs in a proton-transfer reaction, which is the foundational skill. Unit 8 is where conjugate acids do the heavy lifting. LO 8.8.A says a buffer works because the conjugate acid in the pair reacts with any added base (and the conjugate base neutralizes added acid), which is the entire mechanism of pH stabilization. LO 8.9.A builds the Henderson-Hasselbalch equation around the concentration ratio of the conjugate acid-base pair, and LO 8.6.A ties conjugate acid/base strength to molecular structure (strong bases like Group I hydroxides have very weak conjugate acids). If you can't spot the conjugate acid in a reaction, most of Unit 8 falls apart.
Keep studying AP Chemistry Unit 8
Conjugate Base (Units 4 & 8)
The other half of the pair. A conjugate base is the acid minus a proton, so HNO₂ and NO₂⁻ are mirror images of each other. Identify one and you've automatically identified the other, since the two species in a pair differ by exactly one H⁺.
Properties of Buffers (Unit 8)
A buffer is just large amounts of both members of a conjugate acid-base pair living in one solution. When you dump in NaOH, the conjugate acid steps up and neutralizes it. That's why an NH₃/NH₄⁺ mixture resists pH change but pure NH₃ does not.
Henderson-Hasselbalch Equation (Unit 8)
pH = pKa + log([A⁻]/[HA]) is really a statement about the conjugate pair ratio. When the conjugate acid and conjugate base are equimolar, the log term is zero and pH equals pKa, which is exactly what happens at the half-equivalence point of a titration.
Acid-Base Titrations (Units 4 & 8)
At the equivalence point of a weak base titration, all the base has been converted into its conjugate acid. That conjugate acid then reacts with water, which is why the equivalence point pH is below 7 instead of neutral.
The most basic ask is identification. The 2026 Long FRQ Q3 opened by giving the ionization of nitrous acid (HNO₂ + H₂O ⇌ NO₂⁻ + H₃O⁺) and asking you to identify a conjugate acid-base pair, which is free points if you remember the pairs differ by one H⁺. Multiple-choice questions go further. A classic stem gives you a buffer like H₂PO₄⁻/HPO₄²⁻ and asks which net ionic equation shows the buffer responding to added HCl, so you have to know the conjugate base eats the added acid and becomes the conjugate acid. Another common stem gives a reaction like H₂PO₄⁻ + NH₃ ⇌ HPO₄²⁻ + NH₄⁺ and asks you to match each species to its conjugate partner, where amphiprotic species like H₂PO₄⁻ can trip you up. You also use conjugate acids quantitatively in Henderson-Hasselbalch calculations and in finding the pH at the equivalence point of weak acid/base titrations.
They're directional opposites in the same pair. The conjugate acid is what a BASE becomes after gaining H⁺ (NH₃ → NH₄⁺), while the conjugate base is what an ACID becomes after losing H⁺ (HNO₂ → NO₂⁻). A quick check helps. The conjugate acid always has one more H and one more positive charge than its partner. Watch out for amphiprotic species like H₂PO₄⁻, which is the conjugate acid of HPO₄²⁻ but the conjugate base of H₃PO₄, so the label depends on which reaction you're looking at.
A conjugate acid is formed when a Brønsted-Lowry base accepts a proton, so it has one more H and one more unit of positive charge than the base.
Every acid-base reaction contains two conjugate acid-base pairs, and the species in each pair differ by exactly one H⁺.
Strength flips across a pair. Strong bases like Group I and II hydroxides have very weak conjugate acids, while weak bases like NH₃ have conjugate acids that matter in equilibrium calculations.
Buffers work because the conjugate acid in the pair neutralizes added base while the conjugate base neutralizes added acid, which is the mechanism behind LO 8.8.A.
In the Henderson-Hasselbalch equation, pH equals pKa when the conjugate acid and conjugate base concentrations are equal, which happens at the half-equivalence point.
Amphiprotic species like H₂O and H₂PO₄⁻ can act as a conjugate acid in one reaction and a conjugate base in another, so always check the specific equation.
It's the species formed when a Brønsted-Lowry base accepts a proton (H⁺). For example, NH₄⁺ is the conjugate acid of NH₃, and H₃O⁺ is the conjugate acid of H₂O.
A conjugate acid is a base plus a proton, while a conjugate base is an acid minus a proton. In HNO₂ + H₂O ⇌ NO₂⁻ + H₃O⁺, NO₂⁻ is the conjugate base of HNO₂ and H₃O⁺ is the conjugate acid of H₂O.
No, it's the opposite. The CED states that strong bases like Group I and II hydroxides have very weak conjugate acids. Strength is inversely related across a conjugate pair, so a weaker base produces a stronger conjugate acid.
Add one H to the formula and add one to the charge. So HPO₄²⁻ becomes H₂PO₄⁻, NH₃ becomes NH₄⁺, and OH⁻ becomes H₂O. That single-proton difference is the only thing separating the two members of a pair.
Per LO 8.8.A, a buffer contains large amounts of both members of a conjugate acid-base pair. The conjugate acid is the component that reacts with any added base, so without it the solution couldn't resist pH increases.