Acids and bases are everywhere in chemistry and in life. Unit 8 teaches you how to identify acids and bases, calculate pH and pOH, understand strong and weak acids, and solve problems involving buffers and titrations. You'll learn the relationship between acid and base strength and molecular structure. These concepts connect directly to equilibrium from Unit 7 and to electrochemistry in Unit 9.
🎯 What You Need to Know for the Exam
Unit 8 makes up about 11-15% of the AP Chemistry exam. This is one of the heavier units. Focus your energy on these priorities:
What's in this review:
Water is the solvent in most acid-base chemistry. Water has a remarkable property: it undergoes autoionization, producing hydronium ions (H3O+) and hydroxide ions (OH-). In pure water, the concentration of H3O+ equals the concentration of OH-. The product of these concentrations is constant at a given temperature, called Kw.
At 25°C, Kw = [H3O+][OH-] = 1.0 × 10^-14. This is the fundamental equation of water equilibrium. It means that if you know the concentration of hydronium ions, you can calculate the concentration of hydroxide ions, and vice versa.
We measure acidity using pH, defined as pH = -log[H3O+]. Similarly, pOH = -log[OH-]. A key relationship is pH + pOH = 14.0 at 25°C. A neutral solution has pH = 7 and pOH = 7. Acidic solutions have pH < 7. Basic solutions have pH > 7.
Key concepts to know:
⚠ Watch out for:
Don't forget that Kw changes with temperature. At temperatures other than 25°C, the pH of pure neutral water deviates from 7.0. The exam sometimes includes this detail. Also, remember that pH + pOH = 14.0 only at 25°C. At other temperatures, use the actual Kw value.
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Topic
AP Chemistry: Water Equilibrium and pH
Focus on
Kw, pH and pOH calculations, neutral solutions, autoionization
📝 Quiz · 15 questions
Topic
AP Chemistry: Water Equilibrium and pH
Description
Calculate pH and pOH, use Kw relationships, identify acidic and basic solutions
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Strong acids completely ionize in water. When you dissolve a strong acid like HCl, it dissociates into H+ and Cl-. The concentration of H3O+ equals the initial concentration of the acid. If you dissolve 0.1 M HCl, you get [H3O+] = 0.1 M, and pH = 1.
Strong bases like NaOH completely dissociate. When you dissolve a strong base, it produces OH- ions. A 0.1 M NaOH solution gives [OH-] = 0.1 M. But be careful: for Group II hydroxides like Ca(OH)2, each formula unit produces two OH- ions. A 0.1 M Ca(OH)2 solution gives [OH-] = 0.2 M.
Common strong acids include HCl, HBr, HI, HClO4, H2SO4, and HNO3. Common strong bases include Group I and Group II hydroxides. If the acid or base is not on the strong list, treat it as weak.
Key concepts to know:
⚠ Watch out for:
Remember that H2SO4 is a strong acid in its first dissociation but the second dissociation is weak. For typical problems, treat it as strong. Also, for Group II bases, don't forget to multiply by 2. A 0.1 M Ba(OH)2 solution has [OH-] = 0.2 M, not 0.1 M.
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🃏 Flashcards · 15 cards
Topic
AP Chemistry: Strong Acids and Bases pH
Focus on
Complete ionization, strong acid list, strong base list, Group II hydroxides
📝 Quiz · 15 questions
Topic
AP Chemistry: Strong Acids and Bases pH
Description
Calculate pH from strong acid concentration, calculate pOH from strong base concentration
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Weak acids only partially ionize. A weak acid HA in water establishes an equilibrium: HA ⇌ H+ + A-. The equilibrium constant for this reaction is Ka (the acid ionization constant). Only a small fraction of the acid molecules ionize. The vast majority remain as HA molecules.
The relationship is: Ka = [H3O+][A-] / [HA]. You can also express this as pKa = -log Ka. A larger Ka means a stronger acid (ionizes more). A smaller Ka means a weaker acid (ionizes less).
Similarly, weak bases establish an equilibrium: B + H2O ⇌ BH+ + OH-. The equilibrium constant is Kb. The percent ionization describes what fraction of the base (or acid) ionizes. It's calculated from the ratio of ionized to initial concentration, multiplied by 100%.
For a conjugate acid-base pair, Ka × Kb = Kw. This is a critical relationship. If you know Ka for an acid, you can find Kb for its conjugate base.
Key concepts to know:
⚠ Watch out for:
Weak acid problems require equilibrium setup. Don't treat them like strong acids. You need Ka or Kb to find the pH. Also, remember that percent ionization is not the same as the degree of ionization. Know both definitions. And don't confuse Ka with Kb. Ka is for acids, Kb is for bases.
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🃏 Flashcards · 20 cards
Topic
AP Chemistry: Weak Acids and Bases
Focus on
Ka and Kb expressions, percent ionization, conjugate pairs, pKa and strength
📝 Quiz · 20 questions
Topic
AP Chemistry: Weak Acids and Bases
Description
Set up weak acid/base equilibria, calculate pH, determine percent ionization
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When you mix acids and bases, they react. A strong acid and strong base produce water: H+ + OH- → H2O. The pH of the resulting solution depends on which reactant was in excess. If the acid is in excess, the solution is acidic. If the base is in excess, the solution is basic.
When a weak acid and strong base mix, they form a buffer solution if the weak acid is in excess. A buffer contains both the weak acid (HA) and its conjugate base (A-). The buffer resists pH changes when small amounts of acid or base are added.
When a weak base and strong acid mix, a buffer forms if the weak base is in excess. The buffer contains the weak base (B) and its conjugate acid (BH+).
When two weak species mix, the reaction reaches equilibrium. Calculate which species is present in greater concentration, then use its Ka or Kb to find pH.
Key concepts to know:
⚠ Watch out for:
When a weak acid and strong base mix, the quantitative reaction (stoichiometry) happens first. The strong base consumes the weak acid. Then, if excess weak acid remains, the weak acid-conjugate base equilibrium takes over. Track which species dominates after the reaction is complete.
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Topic
AP Chemistry: Acid-Base Reactions
Focus on
Reaction products, buffer formation, calculating pH after mixing
📝 Quiz · 15 questions
Topic
AP Chemistry: Acid-Base Reactions
Description
Predict products, identify buffers, calculate pH after mixing
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A titration is a controlled experiment where you add acid (or base) from a burette to a flask containing the other reactant. You measure the volume of titrant added when the reaction reaches the equivalence point, where moles of acid equal moles of base.
A titration curve plots pH against the volume of titrant added. The curve has a distinctive shape. At the beginning, pH changes slowly. Near the equivalence point, pH changes rapidly (this is the steep part). After the equivalence point, pH changes slowly again.
For a weak acid titrated with strong base, the equivalence point occurs at a pH above 7 (basic). This is because the conjugate base of the weak acid hydrolyzes and makes the solution basic. The half-equivalence point (halfway to the equivalence point) is useful: at this point, pH = pKa of the weak acid.
For monoprotic acids or bases, moles of titrant at equivalence point equals moles of analyte. You can use this to calculate the concentration of the unknown acid or base.
For polyprotic acids (like H2SO4 or H3PO4), the titration curve shows multiple equivalence points, one for each proton. Each equivalence point requires additional titrant. Polyprotic acid titration curves have multiple steep regions, and each half-equivalence point corresponds to a different pKa.
Key concepts to know:
⚠ Watch out for:
The equivalence point is not always pH = 7. It depends on the type of acid and base being titrated. For weak acid titrations, the equivalence point pH is determined by the hydrolysis of the conjugate base. Don't assume equivalence = neutral.
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Topic
AP Chemistry: Titrations
Focus on
Equivalence point, titration curves, half-equivalence point, pH at equivalence
📝 Quiz · 15 questions
Topic
AP Chemistry: Titrations
Description
Interpret titration curves, calculate analyte concentration, find equivalence point pH
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The strength of an acid or base relates to molecular structure. A strong acid has a very weak conjugate base, meaning the conjugate base is stabilized. Strong acids have protons on atoms that are electronegative, or they have structures where the negative charge on the conjugate base is spread out (resonance stabilization).
Carboxylic acids (R-COOH) are a common weak acid class. The OH in the carbonyl group is more acidic than an ordinary hydroxyl because the negative charge on the conjugate carboxylate anion is stabilized by resonance. The O=C-O- structure distributes the negative charge over two oxygens.
Strong bases have very weak conjugate acids. Group I and II hydroxides are strong bases. The conjugate acids (water and water, respectively) are very weak. Weak bases include ammonia (NH3) and amines, where nitrogen has a lone pair available for accepting protons.
Key concepts to know:
⚠ Watch out for:
Don't assume that a molecule with many OH groups is a strong base. Electronegativity, resonance, and inductive effects determine strength. NH3 is a weak base even though it's a Brønsted base. And remember that the carboxylate ion (COO-) is a weak base, while carboxylic acid is a weak acid.
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Topic
AP Chemistry: Acid-Base Structure and Strength
Focus on
Electronegativity effects, resonance stabilization, carboxylic acids, strong acid characteristics
📝 Quiz · 10 questions
Topic
AP Chemistry: Acid-Base Structure and Strength
Description
Predict acid/base strength from structure, explain effects of resonance and electronegativity
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The relationship between pH and pKa tells you the protonation state of an acid or base. When pH < pKa, the acid form (HA) is more abundant. When pH > pKa, the base form (A-) is more abundant. When pH = pKa, the two forms are equal in concentration.
This is a powerful tool. If you know the pKa of an acid and the pH of the solution, you can predict which form dominates without calculating. For example, if pKa of acetic acid is 4.74, then at pH 3, acetic acid (HA) dominates. At pH 6, acetate ion (A-) dominates.
Acid-base indicators are molecules that have different colors in their protonated and deprotonated states. The color changes when pH crosses the pKa of the indicator. To choose an indicator for a titration, select one whose pKa is close to the equivalence point pH.
Key concepts to know:
⚠ Watch out for:
The pH vs. pKa relationship works for weak acids and their conjugate bases. For strong acids, pKa is very negative, so pH > pKa always holds. Don't try to use this relationship for strong acids.
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Topic
AP Chemistry: pH, pKa, and Protonation State
Focus on
pH vs. pKa relationships, predicting dominant form, indicator selection
📝 Quiz · 10 questions
Topic
AP Chemistry: pH, pKa, and Protonation State
Description
Use pH and pKa to predict forms, select indicators for titrations
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A buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid) in significant concentrations. The buffer resists pH changes when small amounts of acid or base are added. When you add a small amount of strong acid to a buffer, the conjugate base reacts with the added H+, removing it from solution and preventing a large pH drop. When you add a small amount of strong base, the weak acid reacts with the added OH-, removing it from solution and preventing a large pH rise.
Buffers don't prevent pH change entirely. They reduce the change. A buffer is effective when pH is within about one unit of the pKa of the weak acid. Outside this range, the buffer capacity becomes poor.
Key concepts to know:
⚠ Watch out for:
A buffer requires both the acid and base forms in significant concentrations. Just having a weak acid or just having the conjugate base doesn't make a buffer. Both must be present. Also, buffers don't prevent pH change, they reduce it.
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Topic
AP Chemistry: Buffer Solutions
Focus on
Buffer composition, pH resistance mechanisms, effective buffer ranges
📝 Quiz · 10 questions
Topic
AP Chemistry: Buffer Solutions
Description
Identify buffers, explain how they work, determine buffer effectiveness
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The Henderson-Hasselbalch equation is a shortcut for calculating the pH of a buffer:
pH = pKa + log([A-]/[HA])
This equation comes directly from the Ka expression and is used to quickly find the pH when you know the pKa and the ratio of conjugate base to acid concentrations. If the concentrations of A- and HA are equal, the log term is zero and pH = pKa. If A- is 10 times more concentrated than HA, the pH is about 1 unit higher than pKa.
The equation is particularly useful for buffer problems. When you add acid or base to a buffer, the ratio [A-]/[HA] changes slightly, but the Henderson-Hasselbalch equation lets you calculate the new pH quickly. Adding a small amount of strong acid consumes some A-, so the ratio decreases and pH drops slightly. Adding a small amount of strong base consumes some HA, so the ratio increases and pH rises slightly.
Key concepts to know:
⚠ Watch out for:
The Henderson-Hasselbalch equation assumes that the pH is within the buffer's effective range and that adding acid or base doesn't dramatically change the concentrations. Also, use pKa, not Ka, in the equation. And remember the log is base 10, not natural log. Note: The CED excludes calculations of the exact pH change when strong acid or base is added to a buffer. Focus on qualitative predictions of how the ratio and pH shift.
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Topic
AP Chemistry: Henderson-Hasselbalch Equation
Focus on
Using the equation, calculating pH from ratio, interpreting log term
📝 Quiz · 10 questions
Topic
AP Chemistry: Henderson-Hasselbalch Equation
Description
Calculate buffer pH, predict pH changes with additions
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Buffer capacity is the amount of acid or base a buffer can neutralize while maintaining a relatively constant pH. A buffer with high capacity can neutralize a large amount of added acid or base. A buffer with low capacity can only handle small additions.
Buffer capacity depends on the concentrations of the weak acid and conjugate base. A buffer made with high concentrations of HA and A- has greater capacity than one made with low concentrations (assuming the same ratio). Increasing the concentration of the buffer components increases capacity without changing pH.
The ratio of acid to base also affects capacity asymmetrically. If you have more conjugate base than acid, the buffer has greater capacity for neutralizing added acid. If you have more acid than base, it has greater capacity for neutralizing added base.
Key concepts to know:
⚠ Watch out for:
Buffer capacity is not just about how much acid or base you add; it's about how much the pH changes. A buffer can fail if you add too much and overwhelm its capacity. Also, the ratio of acid to base determines directional capacity, not just total capacity.
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Topic
AP Chemistry: Buffer Capacity
Focus on
Factors affecting capacity, concentration effects, directional capacity
📝 Quiz · 10 questions
Topic
AP Chemistry: Buffer Capacity
Description
Analyze buffer capacity, predict effects of concentration changes
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The solubility of a salt is pH sensitive when one of the ions is a weak acid, weak base, or hydroxide ion. For example, the solubility of CaCO3 increases at low pH. The carbonate ion is the conjugate base of a weak acid (HCO3-). At low pH, H+ ions protonate the CO3^2- ions, forming HCO3- and H2CO3. This removes CO3^2- from the equilibrium, allowing more CaCO3 to dissolve.
Similarly, the solubility of a metal hydroxide like Mg(OH)2 decreases at high pH. Adding OH- shifts the dissolution equilibrium left, precipitating more Mg(OH)2.
These effects can be understood qualitatively using Le Châtelier's principle. The exam does not require quantitative calculations of solubility as a function of pH. Just understand the direction of the effect and the mechanism.
Key concepts to know:
⚠ Watch out for:
The CED specifically excludes quantitative solubility-pH calculations. Know the qualitative effects. Understand why they happen. But don't expect to calculate exact solubility values at different pH values on the AP exam.
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🃏 Flashcards · 10 cards
Topic
AP Chemistry: pH Effects on Solubility
Focus on
pH-sensitive salts, protonation effects, Le Châtelier's principle
📝 Quiz · 10 questions
Topic
AP Chemistry: pH Effects on Solubility
Description
Predict solubility changes with pH, explain using Le Châtelier's principle
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Unit 8 is the most mathematical unit in AP Chemistry. Here are strategies that work:
Memorize the strong acid and base lists. This is non-negotiable. If it's not on the list, it's weak. Spend 5 minutes memorizing: HCl, HBr, HI, HClO4, H2SO4, HNO3 (acids) and Group I and II hydroxides (bases).
Master the weak acid setup. Weak acid problems follow a pattern: write the equilibrium, set up Ka expression, use ICE table, solve for [H+], then pH. Practice this 10 times until it's automatic.
Understand buffers conceptually, then mathematically. First, understand that a buffer is a weak acid and its conjugate base. Then, understand that the Henderson-Hasselbalch equation is a shortcut. Finally, practice applying both the concept and the equation.
Practice titration curve interpretation. Draw the shape from memory: flat at start, then steep near equivalence point, then flat again. Label where pH = pKa, equivalence point pH, and half-equivalence point.
Connect to equilibrium. Unit 8 is just equilibrium applied to acid-base chemistry. Use Le Châtelier's principle to explain buffer behavior, indicator choice, and pH effects on solubility.
StarSpark Practice Prompts:
You've covered all the topics in Unit 8. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.
Review Questions: Test Yourself
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Unit 8 connects equilibrium (Unit 7) to electrochemistry (Unit 9). Understanding acid-base chemistry is essential for both. Buffers are equilibrium systems that resist pH change. Titration curves show you the practical application of equilibrium. And electrochemical cells depend on the oxidation and reduction of species in acidic or basic environments.
Check out the full AP Chemistry study plan to see how this unit connects to the rest of the course.
Other Unit Reviews:
For official AP Chemistry resources, visit apcentral.collegeboard.org.
This review is aligned with the AP Chemistry Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.