AP Chemistry Bronsted-Lowry Acids and Bases

Acids, bases, and conjugate pairs, with four worked examples.

Almost every acid-base idea in AP Chemistry, from buffers to titration curves, starts with one skill: looking at a reaction and saying which species gave up a proton, which one took it, and what each one turns into. The method is the same every time, so the worked examples below cover the cases that cause mistakes.

How to Identify Bronsted-Lowry Acids, Bases, and Conjugate Pairs

A Brønsted-Lowry acid donates a proton (H⁺); a Brønsted-Lowry base accepts one. Each acid and the base it leaves behind form a conjugate pair, and the two members of a pair always differ by exactly one H⁺.

Find which reactant loses H⁺ and which gains it. The donor is the acid, the acceptor is the base.
Write each conjugate. Acid minus H⁺ is its conjugate base (charge down by 1). Base plus H⁺ is its conjugate acid (charge up by 1).
Check each pair differs by exactly one H⁺.

Lewis acid-base ideas are not tested; see the Unit 4 review for how this model fits into acid-base reactions.

Worked Example 1: Hydrofluoric Acid Reacting With Water

Identify the acid, the base, and both conjugate pairs: HF(aq) + H₂O(l) ⇌ F⁻(aq) + H₃O⁺(aq).

HF loses H⁺ and becomes F⁻, so HF is the acid and F⁻ is its conjugate base.
H₂O gains H⁺ and becomes H₃O⁺, so H₂O is the base and H₃O⁺ is its conjugate acid.
Pairs: HF / F⁻ and H₃O⁺ / H₂O.

Worked Example 2: Ammonia Reacting With Water

Identify the acid, the base, and both conjugate pairs: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).

NH₃ gains H⁺ and becomes NH₄⁺, so NH₃ is the base and NH₄⁺ is its conjugate acid.
H₂O loses H⁺ and becomes OH⁻, so H₂O is the acid and OH⁻ is its conjugate base.
Pairs: NH₄⁺ / NH₃ and H₂O / OH⁻.

Compare with Example 1: water was the base there and is the acid here. Water can play either role, which is why it is called amphiprotic.

Worked Example 3: The Amphiprotic Bicarbonate Ion

Show that HCO₃⁻ can act as both an acid and a base with water.

As an acid (donates H⁺): HCO₃⁻ + H₂O ⇌ CO₃²⁻ + H₃O⁺. The conjugate base is CO₃²⁻ (one H⁺ removed, charge down by 1).
As a base (accepts H⁺): HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻. The conjugate acid is H₂CO₃ (one H⁺ added, charge up by 1).

Any species that still has a removable H⁺ and can also accept one, such as HCO₃⁻ or H₂O, is amphiprotic. This is the same conjugate-pair idea behind a buffer.

Worked Example 4: Linking Acid Strength to Its Conjugate Base

Acetic acid, CH₃COOH, has K_a = 1.8 × 10⁻⁵. How basic is its conjugate base, acetate?

The strength of a conjugate pair flips: the stronger the acid, the weaker its conjugate base. HCl is a strong acid, so Cl⁻ is a negligibly weak base. Acetic acid is weak, so acetate is a weak base, not a negligible one, and the two constants are linked by K_a × K_b = K_w:
K_b = K_w / K_a = (1.0 × 10⁻¹⁴) / (1.8 × 10⁻⁵) = 5.6 × 10⁻¹⁰

This is the same K_b used for acetate at the equivalence point in the titration curve calculations, where it feeds an ICE table.

Common Bronsted-Lowry Mistakes

Removing the wrong thing to make a conjugate. A conjugate base is the acid minus one H⁺, not minus an OH or an entire hydrogen atom, and the charge must drop by exactly 1.
Treating water as always the solvent only. Water is a base in Example 1 and an acid in Example 2; check which way the proton moves.
Assuming a strong acid has a strong conjugate base. It is the reverse: strong acids have negligibly weak conjugate bases.
Pairing the wrong species. Each conjugate pair sits on opposite sides of the arrow and differs by exactly one H⁺.

This same proton-transfer reasoning underlies every acid-base topic in AP Chemistry, including Unit 8 -- for every other free tool and guide on this site, start from the AP Chem Score Calculator.

Frequently Asked Questions

What is a Bronsted-Lowry acid and base?

A Bronsted-Lowry acid is a proton (H+) donor, and a Bronsted-Lowry base is a proton acceptor. Every acid-base reaction in this model is a proton transfer.

What is a conjugate acid-base pair?

Two species that differ by exactly one H+. When an acid donates a proton, what is left is its conjugate base; when a base accepts a proton, what it becomes is its conjugate acid. Every proton-transfer reaction contains two conjugate pairs.

How do you find the conjugate base of an acid?

Remove one H+: take away one hydrogen and reduce the charge by 1. HF becomes F-, H2O becomes OH-, and HCO3- becomes CO3(2-).

What does amphiprotic mean?

A species that can act as either an acid or a base depending on what it reacts with. Water is the most common example, and HCO3- is another: it can donate a proton to become CO3(2-), or accept one to become H2CO3.

Do I need to know Lewis acids and bases for AP Chemistry?

No. Lewis acid-base concepts (electron-pair donors and acceptors) are explicitly excluded. AP Chemistry stays on Bronsted-Lowry acids and bases in aqueous solution.

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