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Acid-Base Reactions and Buffers

When an acid and base react, they neutralize, and the resulting pH depends on what remains. A buffer — a weak acid and its conjugate base together — resists pH change: added acid is absorbed by the conjugate base, and added base by the weak acid.

UNIT 8 TOPIC 8.4 • ACID-BASE REACTIONS AND BUFFERS BUFFER ACTION A buffer holds appreciable amounts of BOTH a weak acid (HA) and its conjugate base (A⁻) that coexist in solution. BUFFER RESERVOIR weak acid HA HA HA HA HA conjugate base A⁻ A⁻ A⁻ A⁻ A⁻ Both members of the conjugate pair are present at the same time. ADD STRONG ACID (extra H⁺) A⁻ + H⁺HA The conjugate base soaks up the added H⁺, forming HA. ADD STRONG BASE (extra OH⁻) HA + OH⁻A⁻ + H₂O The weak acid soaks up the added OH⁻, forming A⁻ and water. RESULT The pH changes far less than it would in pure water — the buffer resists change. CED ANCHOR Buffers keep pH nearly constant by using an equilibrium reservoir of a conjugate acid–base pair (TRA-3): each member neutralizes one type of added strong species. A buffer can equally well be made from a weak base and its conjugate acid. AP Chemistry · Unit 8 · Acids and Bases
A buffer is a weak acid paired with its conjugate base (or a weak base with its conjugate acid). It resists pH change: added acid is neutralized by the conjugate base, and added base by the weak acid. Predicting the pH after a neutralization requires tracking what remains.
Buffer Builder · Open the sandbox →

The trap mispredicts the pH after neutralization — for instance, assuming the mixture is neutral without checking whether excess acid or base (or a buffer) remains. Track the moles of each species after the reaction to find the pH.

The work

3 ways in · any order
Lesson
Reactions and Buffers

Neutralization's pH depends on what remains, and a buffer (weak acid + conjugate base) resists pH change. The lesson tracks the leftover species, then closes with a ten-scenario check.

Skill check · 10 scenarios
Diagnostic
10-item topic check

Ten items spanning the Topic 8.4 misconception: the pH after a neutralization mispredicted by not tracking the excess or buffer species that remain.

Not started · 10 items · ~15 min
Targeted Practice
Drill a single misconception

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions