Write the equation for how a buffer of NH_3 and NH_4^+ neutralizes added strong base (OH^-). [2 points]

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What happens when acids and bases react, and how does a buffer resist changes in pH?

Topic 8.4 Acid-Base Reactions and Buffers: predict the products of acid-base reactions, identify the salts formed, and explain how a buffer made from a weak acid and its conjugate base resists pH change.

A focused answer to AP Chemistry Topic 8.4, covering neutralisation reactions and the salts produced, the composition of a buffer, and how a buffer of a weak acid and its conjugate base resists pH change, with full worked examples.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-04

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Quick answer

An acid-base (neutralisation) reaction transfers a proton from the acid to the base, producing water and a salt. The salt's ions may themselves be acidic, basic or neutral, which affects the pH of the resulting solution. A buffer is a solution that resists changes in pH; it contains a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable amounts. When a small amount of strong acid is added, the conjugate base neutralizes it; when a small amount of strong base is added, the weak acid neutralizes it. Because the buffer holds a large reservoir of both components, the added acid or base is converted into the other component, changing their ratio (and so the pH) only slightly. A buffer works best when the weak acid and conjugate base are present in similar amounts.

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What this topic is asking
Acid-base reactions and the salts formed
What a buffer is
How a buffer resists pH change
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What this topic is asking

The College Board (Topic 8.4) wants you to predict the products of acid-base reactions, identify the salts formed, and explain how a buffer made from a weak acid and its conjugate base resists pH change. Buffers are a recurring exam theme and the conceptual heart of this topic.

Acid-base reactions and the salts formed

For a strong acid with a strong base, the salt is neutral and the solution is neutral at the equivalence point. For a weak acid with a strong base, the salt's anion is a weak base, so the solution is basic. Predicting the products means combining the cation and anion and recognizing whether the resulting ions hydrolyze.

What a buffer is

A buffer requires both a proton donor (the weak acid) and a proton acceptor (the conjugate base) present together. A strong acid cannot buffer because it is fully ionized and has no reservoir of un-ionized acid. The two components are usually prepared by mixing a weak acid with a salt of its conjugate base.

How a buffer resists pH change

When a small amount of strong acid ( $\text{H}^+$ ) is added, the conjugate base mops it up: $\text{A}^- + \text{H}^+ \rightarrow \text{HA}$ . When a small amount of strong base ( $\text{OH}^-$ ) is added, the weak acid neutralizes it: $\text{HA} + \text{OH}^- \rightarrow \text{A}^- + \text{H}_2\text{O}$ . Because the buffer holds a large reservoir of both HA and $\text{A}^-$ , the added acid or base is converted into the other component, so the ratio $\dfrac{[\text{A}^-]}{[\text{HA}]}$ changes only slightly, and the pH (which depends on that ratio, Topic 8.7) barely moves. The buffer is most effective when the two components are in similar amounts.

How a buffer absorbs added acid

A buffer contains $0.40$ mol of weak acid HA and $0.40$ mol of conjugate base $\text{A}^-$ in $1.0$ L. Trace what happens when $0.050$ mol of strong acid is added.

step 1 Identify the reacting component

Added $\text{H}^+$ reacts with the conjugate base: $\text{A}^- + \text{H}^+ \rightarrow \text{HA}$ .

step 2 Update the amounts

$\text{A}^-$ falls by $0.050$ to $0.35$ mol; HA rises by $0.050$ to $0.45$ mol.

step 3 Compare the ratio before and after

Before: $\dfrac{0.40}{0.40} = 1.0$ . After: $\dfrac{0.35}{0.45} = 0.78$ .

step 4 Interpret the pH change

The ratio changed only modestly, so the pH changes only slightly; without the buffer, the same $0.050$ mol of strong acid would have dropped the pH sharply.

Try this

Q1. Write the equation for how a buffer of $\text{NH}_3$ and $\text{NH}_4^+$ neutralizes added strong base ( $\text{OH}^-$ ). [2 points]

Cue. $\text{NH}_4^+ + \text{OH}^- \rightarrow \text{NH}_3 + \text{H}_2\text{O}$ (the conjugate acid neutralizes the added base).

Q2. Explain why a solution of a strong acid alone cannot act as a buffer. [2 points]

Cue. It is fully ionized, with no reservoir of un-ionized acid or conjugate base to neutralize added acid or base, so it cannot resist pH change.

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 2023 (style)4 marksSection II (long FRQ, part). A buffer is made from

0.50

M acetic acid (

\text{CH}_3\text{COOH}

) and

0.50

M sodium acetate (

\text{CH}_3\text{COONa}

). (a) Identify the weak acid and the conjugate base in this buffer. (b) Write the equation showing how the buffer neutralizes a small amount of added strong acid (

\text{H}^+

). (c) Write the equation showing how it neutralizes a small amount of added strong base (

\text{OH}^-

). (d) Explain why the pH changes only slightly when a small amount of strong acid or base is added.

Show worked answer →

A 4-point conceptual FRQ on buffers.

(a) Components (1 point): the weak acid is acetic acid ( $\text{CH}_3\text{COOH}$ ); the conjugate base is acetate ( $\text{CH}_3\text{COO}^-$ ).
(b) Added acid (1 point): the conjugate base consumes it: $\text{CH}_3\text{COO}^- + \text{H}^+ \rightarrow \text{CH}_3\text{COOH}$ .
(c) Added base (1 point): the weak acid consumes it: $\text{CH}_3\text{COOH} + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{H}_2\text{O}$ .
(d) Resistance (1 point): the buffer contains a large reservoir of both the weak acid and its conjugate base, so added $\text{H}^+$ or $\text{OH}^-$ is converted into the other buffer component, changing the ratio only slightly and so changing the pH only slightly.

Markers reward identifying the components, both neutralisation equations, and the reservoir reasoning for the small pH change.

AP 2021 (style)1 marksSection I (multiple choice). Which combination forms a buffer solution? (A) a strong acid and its salt (B) a weak acid and its conjugate base (C) a strong acid and a strong base (D) two strong acids. Justify your choice.

Show worked answer →

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

A buffer requires a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable amounts, so it can neutralize both added acid and added base. A strong acid fully ionizes and cannot form a buffer. The trap is (A): strong acids do not buffer.

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Sources & how we know this

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