Acids & Bases

What this unit covers

The topics below follow the published Chemistry course framework for Unit 8. This unit is worth 11–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.

Lessons in this unit

• Acids, Bases & the pH Scale12 min · 3 objectivesDefine acids and bases by the Arrhenius and Brønsted–Lowry models · Identify conjugate acid–base pairs in a proton-transfer reaction · Convert freely among [H⁺], [OH⁻], pH, and pOH using Kw
• Weak Acids/Bases: Ka, Kb & pH14 min · 3 objectivesDistinguish strong from weak acids and bases by degree of ionization · Relate Ka, Kb, and pKa to acid strength · Calculate the pH of a weak acid solution using an ICE table
• Buffers & Henderson–Hasselbalch13 min · 3 objectivesExplain how a weak acid and its conjugate base resist pH change · Use the Henderson–Hasselbalch equation to find buffer pH · Predict how buffer pH depends on the conjugate-base-to-acid ratio
• Titration Curves13 min · 3 objectivesRead the key features of strong–strong and weak–strong titration curves · Locate the equivalence and half-equivalence points and interpret each · Use the half-equivalence point to read pKa directly from a curve
• Salt Hydrolysis & Polyprotic Acids14 min · 3 objectivesPredict whether a salt solution is acidic, basic, or neutral from the origin of its ions · Calculate the pH of a salt solution using Kb = Kw/Ka and an ICE table · Explain stepwise dissociation of a polyprotic acid where Ka1 ≫ Ka2
• Titration Curves & Buffer Design — Advanced15 min · 3 objectivesCompute the pH in each region of a weak-acid/strong-base titration curve · Explain why the equivalence point of a weak-acid titration lies above pH 7 · Design a buffer of a target pH by choosing a pKa and computing the [A⁻]/[HA] ratio

Formulas in Unit 8

Every term in Unit 8

All 20 terms we publish for Acids & Bases, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.

Brønsted-Lowry definition

An acid donates a proton and a base accepts one. Conjugate pairs differ by exactly one H⁺.

pH, pOH and their relationship

pH = −log[H⁺], pOH = −log[OH⁻], and pH + pOH = 14 at 25 °C.

Strong vs weak acids

Strong acids ionize completely; weak acids reach an equilibrium. This is about extent of ionization, not concentration.

Ka and acid strength

Larger Ka means a stronger acid. pKa = −log Ka, so a smaller pKa means stronger.

Relationship of Ka and Kb

Ka × Kb = Kw = 1.0 × 10⁻¹⁴ for a conjugate pair. A stronger acid necessarily has a weaker conjugate base.

Percent ionization

Ionized concentration over initial concentration, times 100. It rises on dilution even though pH also rises.

Buffer

A weak acid and its conjugate base together. Added H⁺ is consumed by the base and added OH⁻ by the acid, so pH barely moves.

Henderson-Hasselbalch equation

pH = pKa + log([A⁻]/[HA]). At the half-equivalence point the ratio is 1, so pH = pKa.

Buffer capacity

How much acid or base a buffer absorbs before failing. Greatest when concentrations are high and the ratio is near 1.

Titration curve of a weak acid

Rises gradually through a buffer region, has an inflection at the half-equivalence point where pH = pKa, and an equivalence point above pH 7.

Equivalence point pH

Strong-strong gives 7; weak acid with strong base gives above 7; weak base with strong acid gives below 7 — the conjugate formed determines it.

Choosing an indicator

The indicator's pKa should be close to the pH at the equivalence point, so its color change coincides with the steep region.

Polyprotic acids

Ionize stepwise with successively smaller Ka values, producing multiple equivalence points on a titration curve.

Structure and acid strength

Strength rises with the electronegativity of the atom bonded to H across a period, and with atom size down a group — HI is stronger than HF.

Autoionization of water

2H₂O ⇌ H₃O⁺ + OH⁻ with Kw = 1.0 × 10⁻¹⁴ at 25 °C. Kw rises with temperature, so neutral pH is below 7 in hot water.

Amphoteric species

Can act as either acid or base — water and HCO₃⁻ are the standard examples.

Salt hydrolysis

The conjugate of a weak acid makes a solution basic; the conjugate of a weak base makes it acidic; conjugates of strong acids and bases are neutral.

Why pH = pKa at half-equivalence

Exactly half the weak acid has been converted to its conjugate base, so [A⁻] = [HA] and the log term in Henderson-Hasselbalch is zero.

Diluting a buffer

pH is nearly unchanged, because dilution divides both [A⁻] and [HA] equally and their ratio is what sets pH. Buffer capacity does fall.

Titration curve features to label

Initial pH, buffer region, half-equivalence point (pH = pKa), equivalence point, and the excess-titrant plateau.

What examiners penalize here

• Identify conjugate pairs by counting protons: the acid always has exactly one more H⁺ than its conjugate base. HCO₃⁻ can be the conjugate base of H₂CO₃ or the conjugate acid of CO₃²⁻ — amphoteric species show up often on the exam.
• Do not confuse concentration with strength. A concentrated weak acid can still have a modest pH, and a dilute strong acid can be nearly neutral. Ka/pKa describes *how completely* an acid ionizes; molarity describes *how much* acid is present.
• A buffer works best when pKa is within about 1 unit of the target pH, i.e. the [A⁻]/[HA] ratio stays between 1:10 and 10:1. To build a buffer for a given pH, choose a weak acid whose pKa is close to that pH.
• Polyprotic acids (H₂CO₃, H₃PO₄) donate protons one at a time, each with its own Ka, so their curves show *multiple* equivalence points — one steep jump per ionizable proton, with Ka1 > Ka2 > Ka3 since each successive H⁺ is harder to pull off a more negative ion.
• On the exam, first sort each salt ion into "spectator" or "hydrolyzes." Group 1/Group 2 cations and the anions of strong acids (Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻) are spectators. Any leftover ion traced to a weak acid or weak base is the one that sets the pH.
• Free-response titration questions reward naming the region before calculating. Say "this is the buffer region, so I use Henderson–Hasselbalch" or "this is past equivalence, so I use excess [OH⁻]." Choosing the right tool for the region earns the method points even before the arithmetic.

Practice Chemistry

Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.

Questions about this unit

All 9 units of AP Chemistry

1. Unit 1 · Atomic Structure & Properties
2. Unit 2 · Molecular and Ionic Compound Structure and Properties
3. Unit 3 · Intermolecular Forces and Properties
4. Unit 4 · Chemical Reactions
5. Unit 5 · Kinetics
6. Unit 6 · Thermodynamics
7. Unit 7 · Equilibrium
8. Unit 8 · Acids & Bases
9. Unit 9 · Applications of Thermodynamics