AP Chemistry, AP exams, 11–15%

Unit 8: Acids and Bases

Unit 8 carries the second largest published weighting in the course. It covers strong and weak acid and base behavior, pH and pOH, acid-base equilibria, titration curves, buffers, and the structural features that make an acid strong. Buffer reasoning and the Henderson-Hasselbalch relationship are the highest-yield skills in the unit.

Position: 8 of 9 units
Published weighting: 11–15% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Chemistry. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Introduction to acids and bases: Bronsted-Lowry definitions, conjugate pairs, and the autoionization of water.
pH and pOH of strong acids and bases: Direct calculation from concentration, and the relationship between pH, pOH, and Kw.
Weak acid and base equilibria: Ka and Kb calculations, percent ionization, and the conjugate relationship between Ka and Kb.
Acid-base reactions and titration curves: Reading equivalence and half-equivalence points, and choosing an appropriate indicator.
Molecular structure of acids and bases: Why bond polarity, bond strength, and conjugate-base stability determine acid strength.
Buffers and buffer capacity: How a conjugate pair resists pH change, the Henderson-Hasselbalch relationship, and what limits capacity.

Study tips

How to practice Acids and Bases.

Section-specific guidance that follows from how this material is usually tested.

Learn to read a titration curve first (strong or weak, which point is which) before calculating anything.
Practice buffer problems both directions: find pH from composition, and find composition needed for a target pH.
Anchor acid strength explanations in conjugate-base stability; that reasoning generalizes better than memorized lists.

Where this fits

Unit 8: Acids and Bases is 8 of 9 units in the AP Chemistry guide, and it carries a published weighting of 11–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Chemistry guide Official exam information

Sample Acids and Bases questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What is the pH of a 0.010 mol/L solution of HCl, a strong acid, at 25 degrees Celsius?
1. A1.00
2. B2.00
3. C7.00
4. D12.00
Show answer
Answer: B. 2.00
A strong acid ionizes completely, so the hydronium ion concentration equals the acid concentration of 0.010 mol/L, and pH is the negative logarithm of that value, which is 2.00. The value 12.00 is the pOH of the same solution, a tempting distractor for anyone who subtracts from 14 out of habit. A pH of 7.00 would describe pure water rather than an acid.
Key insight: For a strong monoprotic acid, the hydronium concentration is simply the stated concentration.
Question 2Foundational
What is the conjugate base of the acid HSO4^-?
1. ASO4^2-
2. BH2SO4
3. CH3O^+
4. DOH^-
Show answer
Answer: A. SO4^2-
A conjugate base is what remains after an acid donates one proton, so removing H+ from HSO4^- leaves SO4^2-. The species H2SO4 is the tempting distractor, but adding a proton produces the conjugate acid, moving in the wrong direction. Conjugate pairs always differ by exactly one H+ and one unit of charge.
Key insight: Subtract one proton for the conjugate base; add one proton for the conjugate acid.
Question 3Intermediate
A 0.10 mol/L solution of a weak monoprotic acid has Ka = 1.0 x 10^-5. What is the approximate pH of the solution?
1. A1.00
2. B3.00
3. C4.00
4. D5.00
Show answer
Answer: B. 3.00
Setting up the equilibrium gives x squared divided by 0.10 equal to 1.0 x 10^-5, since x is small compared with 0.10, so x squared is 1.0 x 10^-6 and x is 1.0 x 10^-3 mol/L. The pH is therefore 3.00. Answering 5.00 reports the pKa instead of the pH, which would only be the pH if the acid and its conjugate base were present in equal amounts.
Key insight: For a weak acid the hydronium concentration is the square root of Ka times the concentration, not Ka itself.
Question 4Intermediate
A buffer is made by dissolving 0.10 mol of a weak acid with pKa 4.75 and 1.0 mol of its conjugate base in 1.0 L of water. What is the pH of the buffer?
1. A3.75
2. B4.75
3. C5.75
4. D9.25
Show answer
Answer: C. 5.75
The Henderson-Hasselbalch relationship gives pH equal to pKa plus the logarithm of the ratio of conjugate base to acid, and that ratio is 1.0 divided by 0.10, or 10, whose logarithm is 1, so the pH is 4.75 plus 1, or 5.75. Answering 4.75 drops the logarithm term; the pH equals the pKa only when the two species are present in equal amounts, which is why the half-equivalence point of a titration reads the pKa. Inverting the ratio gives 3.75, and 9.25 is the pKb of the conjugate base rather than a pH.
Key insight: Each tenfold excess of conjugate base over weak acid raises a buffer's pH one unit above the pKa, and a tenfold excess of acid lowers it one unit.
Question 5Advanced
Fluorine is more electronegative than chlorine, yet HF is a weak acid in water while HCl is a strong acid. What accounts for this?
1. AFluoride is a weaker base than chloride, so HF holds its proton
2. BHF is nonpolar while HCl is polar
3. CThe H-F bond is much stronger, so its proton is harder to release
4. DFluorine has a smaller nuclear charge than chlorine
Show answer
Answer: C. The H-F bond is much stronger, so its proton is harder to release
Within a group, acid strength for binary acids is governed mainly by bond strength rather than electronegativity, and the short H-F bond is the strongest of the hydrogen halides. Because that bond resists breaking, HF ionizes only partially and behaves as a weak acid. The statement about fluoride being a weaker base is the reverse of the truth, since a weak acid by definition has a relatively strong conjugate base.
Key insight: Down a group, bond strength decides acid strength; across a period, electronegativity does.

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Unit 8: Acids and Bases