AP Chemistry Unit 8 Practice

Instructions

  1. Choose a topic, then start. Mixed review includes all eleven topics.
  2. Select one answer, or enter a number and choose its unit. For pH, pOH, pKa and equilibrium constants, choose no units.
  3. Submit once per question. Read the explanation, then continue. Your score counts correct answers; a significant figure note does not remove a point.
  4. At the end, Try again shuffles the questions and answer choices. Choose another topic to build a new set.

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AP Chemistry · Unit 8

Catch up at your own pace.

Choose one topic for six questions, or Mixed review for all 66. Read the feedback after each answer.

No timer. Use scratch paper and a calculator. The acid-base reference is below.

Reference

Acids, bases and logarithms

Use ideal dilute solutions. Unless stated otherwise, temperature is 25 °C. Hydronium is the proton-bearing ion in water.

Kw = [H3O+][OH−] = 1.0 × 10−14 (25 °C)

pH = −log[H3O+]

pOH = −log[OH−]

pH + pOH = 14.00 (25 °C)

Ka × Kb = Kw

pKa = −log Ka

Neutral water has equal hydronium and hydroxide concentrations. Neutral pH depends on temperature because Kw depends on temperature.

Precision and units

Log rule: decimal places in pH, pOH or pKa equal significant figures in the concentration or K value. Keep extra digits until the final answer.

Choose no units for pH, pOH, pKa, Ka, Kb and Kw. Choose M for concentrations and molar solubility. Precision feedback is advisory; a correct value with the right unit earns its point.

Weak-acid and weak-base equilibria

HA(aq) + H2O(l) ⇌ H3O+(aq) + A−(aq)

Ka = [H3O+][A−]/[HA]

NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH−(aq)

Kb = [NH4+][OH−]/[NH3]

An ICE table lists initial, change and equilibrium concentrations. For a single ionizing weak acid or base with initial concentration C and no initial products:

K = x2/(C − x)

For the small-x approximation, check x/C × 100 < 5%. Otherwise solve the full equation. Percent ionization increases on dilution of a weak acid at fixed temperature.

Percent ionization = (ionized acid concentration / initial acid concentration) × 100%.

Buffers and titrations

pH = pKa + log([A−]/[HA])

A−(aq) + H3O+(aq) → HA(aq) + H2O(l)

HA(aq) + OH−(aq) → A−(aq) + H2O(l)

React added strong acid or base completely first. Use remaining conjugate amounts in the buffer ratio. Both components must remain appreciable. Dilution nearly preserves pH but lowers capacity per volume. Greater total concentration increases capacity; a ratio near unity balances acid and base capacity.

At weak-acid half-equivalence, pH = pKa. At equivalence, a weak acid with strong base gives pH above 7; a weak base with strong acid gives pH below 7; a strong acid with strong base gives pH 7.00 at 25 °C. The indicator endpoint is an observed color change near the stoichiometric equivalence point.

Constants used in these questions

Ka and Kb at 25 °C
SpeciesKValue
Acetic acid (CH3COOH)Ka1.8 × 10−5
Acetic acid, more precise measurement (CH3COOH)Ka1.80 × 10−5
Ammonia (NH3)Kb1.8 × 10−5
Hydrofluoric acid (HF)Ka6.8 × 10−4
Acetate, derived (CH3COO−)Kb5.6 × 10−10
Hypothetical weak acid (HA)Ka1.0 × 10−3

pH and solubility

Acid can consume basic anions such as carbonate, fluoride and hydroxide, increasing molar solubility at fixed Ksp. Chloride is not appreciably protonated by dilute acid. Use the free-ion concentrations in Ksp.

CaF2(s) ⇌ Ca2+(aq) + 2F−(aq); Ksp = 3.20 × 10−11

Mg(OH)2(s) ⇌ Mg2+(aq) + 2OH−(aq); Ksp = 5.6 × 10−12

Aligned practice for AP Chemistry CED topics 8.1 to 8.11. Original ZornScience questions.

About this activity

Review AP Chemistry Unit 8, Acids and Bases, with 66 original questions. Each answer gets an explanation right away, and your score is broken down by topic.

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