Give the conjugate acid of the base NH_3. [1 point]

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What is the Bronsted-Lowry model of acids and bases, and how does it identify conjugate pairs?

Topic 8.1 Introduction to Acids and Bases: identify Bronsted-Lowry acids, bases and conjugate acid-base pairs, and distinguish strong from weak acids and bases.

A focused answer to AP Chemistry Topic 8.1, covering the Bronsted-Lowry definitions of acids and bases, conjugate acid-base pairs, amphoteric species, and the distinction between strong and weak acids and bases, with full worked examples.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-04

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
The Bronsted-Lowry definitions
Conjugate acid-base pairs
Amphoteric species and strength
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What this topic is asking

The College Board (Topic 8.1) wants you to identify Bronsted-Lowry acids, bases and conjugate acid-base pairs, and to distinguish strong from weak acids and bases. This is the conceptual foundation of the whole acids-and-bases unit, which is the most heavily weighted unit on the exam.

The Bronsted-Lowry definitions

This model is more general than thinking of acids as producing $\text{H}^+$ and bases as producing $\text{OH}^-$ : it focuses on proton transfer, so it covers reactions in which no hydroxide is involved (such as ammonia accepting a proton from water). The proton is the unit of currency in every acid-base reaction.

Conjugate acid-base pairs

So in $\text{HA} + \text{B} \rightleftharpoons \text{A}^- + \text{HB}^+$ , the pairs are $\text{HA}/\text{A}^-$ and $\text{B}/\text{HB}^+$ . To find a conjugate base, remove one proton; to find a conjugate acid, add one. The charge changes by one unit accordingly.

Amphoteric species and strength

Some species are amphoteric (or amphiprotic), able to act as an acid or a base depending on what they react with. Water is the prime example: it donates a proton to ammonia (acting as an acid) but accepts a proton from hydrochloric acid (acting as a base). Hydrogen carbonate $\text{HCO}_3^-$ behaves similarly.

Acids and bases are also classified by strength. A strong acid or base ionizes essentially completely in water, written with a single arrow; a weak acid or base ionizes only partially and sits at equilibrium, written with a double arrow. Strength is about the extent of ionization, not concentration. There is an inverse relationship: a strong acid has a very weak conjugate base, and a weak acid has a relatively stronger conjugate base.

Identifying acids, bases and conjugates

For $\text{HF}(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{F}^-(aq) + \text{H}_3\text{O}^+(aq)$ , identify the acid, base and conjugate pairs.

step 1 Find the proton donor

HF gives a proton to water, so HF is the acid.

step 2 Find the proton acceptor

Water accepts the proton, so $\text{H}_2\text{O}$ is the base.

step 3 Identify the conjugates

HF loses a proton to become $\text{F}^-$ (its conjugate base); water gains a proton to become $\text{H}_3\text{O}^+$ (its conjugate acid).

step 4 State the pairs and strength

Pairs: $\text{HF}/\text{F}^-$ and $\text{H}_2\text{O}/\text{H}_3\text{O}^+$ . The double arrow shows HF is a weak acid.

Try this

Q1. Give the conjugate acid of the base $\text{NH}_3$ . [1 point]

Cue. $\text{NH}_4^+$ (add one proton).

Q2. Explain the difference between a strong acid and a concentrated acid. [2 points]

Cue. Strength is the extent of ionization (a strong acid ionizes completely); concentration is the amount per liter. A weak acid can be concentrated, and a strong acid can be dilute.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2022 (style)4 marksSection II (long FRQ, part). Consider the reaction

\text{NH}_3(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{NH}_4^+(aq) + \text{OH}^-(aq)

. (a) Identify the Bronsted-Lowry acid and base on the reactant side. (b) Identify the two conjugate acid-base pairs. (c) Explain why water is acting as an acid here even though it can also act as a base. (d) Justify whether

\text{NH}_3

is a strong or weak base, given the double arrow.

Show worked answer →

A 4-point conceptual FRQ on Bronsted-Lowry theory.

(a) Acid and base (1 point): water donates a proton, so $\text{H}_2\text{O}$ is the acid; ammonia accepts a proton, so $\text{NH}_3$ is the base.
(b) Conjugate pairs (1 point): $\text{NH}_3 / \text{NH}_4^+$ (base and its conjugate acid) and $\text{H}_2\text{O} / \text{OH}^-$ (acid and its conjugate base).
(c) Water as an acid (1 point): water is amphoteric; here it donates a proton to the stronger base ammonia, so it acts as the acid in this reaction, though with a strong acid it would accept a proton and act as a base.
(d) Strong or weak (1 point): the double arrow shows the reaction does not go to completion, so $\text{NH}_3$ is a weak base (only partially ionized).

Markers reward identifying the acid and base, both conjugate pairs, the amphoteric reasoning for water, and the weak-base conclusion from the double arrow.

AP 2021 (style)1 marksSection I (multiple choice). The conjugate base of the acid

\text{HCO}_3^-

is (A)

\text{H}_2\text{CO}_3

(B)

\text{CO}_3^{2-}

(C)

\text{CO}_2

(D)

\text{OH}^-

. Justify your choice.

Show worked answer →

A 1-point conceptual MCQ. The answer is (B).

The conjugate base is formed by removing one proton ( $\text{H}^+$ ) from the acid: $\text{HCO}_3^- - \text{H}^+ = \text{CO}_3^{2-}$ . The trap is (A): $\text{H}_2\text{CO}_3$ is the conjugate acid (adding a proton), not the conjugate base.

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Sources & how we know this

AP Chemistry Course and Exam Description — College Board (2020)