Buffers and Acid-Base Titrations Study Pack

Kibin's free study pack on Buffers and Acid-Base Titrations includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

Topic mastery0%
Save progress →

Buffers and Acid-Base Titrations Study Guide

Master the chemistry of buffers and acid-base titrations by working through the Henderson-Hasselbalch equation, buffer effective range, and titration curve analysis. This pack covers equivalence point pH differences for strong and weak acid-base pairs, conjugate base hydrolysis, and indicator selection — everything you need to interpret titration curves and solve buffer problems with confidence.

Key Takeaways

  • •A buffer is a solution that resists changes in pH when small amounts of strong acid or base are added, and it works because it contains both a weak acid and its conjugate base in comparable concentrations.
  • •The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), lets you calculate the pH of a buffer directly from the ratio of conjugate base to weak acid concentrations.
  • •A buffer is most effective when the ratio [A⁻]/[HA] is between 0.1 and 10, meaning the solution pH stays within one unit of the weak acid's pKa.
  • •In an acid-base titration, a solution of known concentration (the titrant) is added to a solution of unknown concentration (the analyte) until the reaction reaches the equivalence point, where moles of acid equal moles of base.
  • •The equivalence point pH depends on the nature of the acid-base pair: strong acid–strong base titrations reach equivalence at pH 7, while weak acid–strong base titrations reach equivalence at a pH greater than 7 due to hydrolysis of the conjugate base.
  • •An indicator signals the endpoint of a titration by changing color near the equivalence point; choosing the right indicator requires matching its pKa to the expected equivalence point pH.
  • •The buffer region of a weak acid titration curve occurs before the equivalence point and is centered at the half-equivalence point, where pH = pKa of the weak acid.

How Buffers Work: Composition and Mechanism

A buffer maintains a relatively stable pH because it contains chemical species capable of neutralizing both added acid and added base without a dramatic shift in hydrogen ion concentration.

Required Components of a Buffer

  • •A buffer must contain a weak acid (HA) and a significant quantity of its conjugate base (A⁻), or equivalently a weak base and its conjugate acid.
  • •The two components are typically present at concentrations within a factor of ten of each other; a ratio far outside 0.1–10 exhausts one component quickly and destroys buffering capacity.
  • •Common buffer systems include acetic acid / sodium acetate (pKa ≈ 4.74), carbonic acid / bicarbonate (pKa ≈ 6.35), and dihydrogen phosphate / hydrogen phosphate (pKa ≈ 7.20).

Neutralization Reactions Inside a Buffer

  • •When a strong acid such as HCl is added, the conjugate base A⁻ consumes the added H⁺ ions: A⁻ + H⁺ → HA. This converts some conjugate base to weak acid but produces no free strong acid.
  • •When a strong base such as NaOH is added, the weak acid HA donates a proton to the hydroxide: HA + OH⁻ → A⁻ + H₂O. This converts some weak acid to conjugate base but consumes the strong base before it can shift pH dramatically.
  • •Because both reactions replace a strong acid or base with a weak one, the change in [H⁺] and therefore pH is small rather than catastrophic.

Buffer Capacity and Its Limits

  • •Buffer capacity refers to the moles of strong acid or base a given volume of buffer can absorb before its pH changes by more than one unit.
  • •Capacity increases with the total concentration of the buffering pair (more HA plus A⁻ available) and is greatest when [A⁻] = [HA], i.e., at the buffer's optimal pH.
  • •Once one component is nearly depleted, adding further acid or base causes a rapid, large pH change, signaling the failure of buffering.

Calculating Buffer pH: The Henderson-Hasselbalch Equation

Quantitative work with buffers centers on the Henderson-Hasselbalch equation, which translates the acid dissociation equilibrium into a practical formula relating pH to the concentration ratio of the conjugate pair.

Deriving the Relationship from Ka

  • •The acid dissociation expression for a weak acid is Ka = [H⁺][A⁻] / [HA]. Solving for [H⁺] gives [H⁺] = Ka × ([HA] / [A⁻]).
  • •Taking the negative logarithm of both sides converts this to pH = pKa + log([A⁻] / [HA]), the Henderson-Hasselbalch equation.
  • •The equation assumes that the weak acid and conjugate base concentrations are not significantly altered by the dissociation equilibrium itself — an assumption valid when concentrations are at least 100 times larger than Ka.

Interpreting the Equation

  • •When [A⁻] = [HA], the log term equals zero, so pH = pKa. This is the half-equivalence point in a titration and represents the center of the effective buffering range.
  • •Increasing the conjugate base fraction relative to the weak acid raises the pH above pKa; decreasing it lowers the pH below pKa.
  • •The effective buffering range spans approximately pKa ± 1, corresponding to [A⁻]/[HA] ratios from 0.1 to 10.

Selecting a Buffer for a Target pH

  • •To prepare a buffer at a desired pH, choose a weak acid whose pKa is within one unit of that target, then adjust the [A⁻]/[HA] ratio using the Henderson-Hasselbalch equation to hit the exact value.
  • •For example, to buffer at pH 5.0 using acetic acid (pKa = 4.74), the equation requires log([A⁻]/[HA]) = 0.26, giving a ratio of about 1.82:1 conjugate base to acid.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

Sources

More in AP Chemistry

See all topics →

Browse other courses

See all courses →