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AP Chemistry › Unit 8

AP Chemistry Unit 8: Acids and Bases

11 to 15% of the exam · 180 practice questions · 5 free-response questions

Unit 8 applies equilibrium to acids and bases. You will calculate pH for strong and weak acids, use Ka and pKa, explain how molecular structure affects acid strength, and analyze buffers with the Henderson-Hasselbalch equation. Titration curves are a major focus: find the equivalence and half-equivalence points, the buffer region, and a suitable indicator. Students often confuse the equivalence point, the half-equivalence point, and the end point. Another common error is forgetting that at half-equivalence the pH equals the pKa of the weak acid.

Practice this unit

White belt · 60 questionsRegular · 60 questionsBlack belt · 60 questionsFree response

White belt covers core terms and foundational ideas, Regular is set at exam level, and Black belt has harder multi-step questions.

Topics in this unit

Free-response questions

Sample questions

These are taken from the Regular set, with the answer and explanation shown.

Topic 8.2 pH and pOH of Strong Acids and Bases

1. What mass of Ba(OH)₂ (molar mass 171.34 g/mol) is needed to make 250.0 mL of a solution with pH 12.00 at 25 °C?

  1. A0.107 g
  2. B0.214 g
  3. C0.428 g
  4. D1.71 g

Answer: B. pH 12.00 means pOH 2.00 and [OH⁻] = 0.010 M. Ba(OH)₂ gives 2 OH⁻, so [Ba(OH)₂] = 0.0050 M. Moles = 0.0050 M × 0.2500 L = 1.25 × 10⁻³ mol, and mass = 1.25 × 10⁻³ × 171.34 = 0.214 g.

Common trap: Choosing 0.428 g, which forgets that each Ba(OH)₂ provides two OH⁻ ions.

Topic 8.6 Molecular Structure of Acids and Bases

The table gives acid ionization constants for six weak acids at 25 °C.

Acid ionization constants at 25 °C
AcidStructureKa
Hypochlorous acidH–O–Cl3.0 × 10⁻⁸
Hypobromous acidH–O–Br2.3 × 10⁻⁹
Hypoiodous acidH–O–I2.3 × 10⁻¹¹
Chlorous acidH–O–Cl=O1.1 × 10⁻²
Acetic acidCH₃COOH1.8 × 10⁻⁵
Chloroacetic acidClCH₂COOH1.4 × 10⁻³

2. Which statement best explains the trend in Ka for HOCl, HOBr and HOI?

  1. AThe O–H bond is longest in HOI, so HOI loses its proton most easily.
  2. BElectronegativity of the halogen decreases from Cl to I, so less electron density is pulled from the O–H bond.
  3. CThe halogen atom gets larger from Cl to I, so the H–X bond gets weaker and breaks more easily.
  4. DThe number of oxygen atoms decreases from HOCl to HOI, so the O–H bond becomes more polar.

Answer: B. In all three acids, the proton is on oxygen and the halogen is bonded to that oxygen. Cl is the most electronegative, so it pulls the most electron density away from the O–H bond, making it more polar. Ka drops as electronegativity drops: HOCl > HOBr > HOI.

Common trap: Choosing the "larger atom, weaker bond" answer. That explains HI > HBr > HCl, where H is bonded directly to the halogen, not these oxoacids.

Topic 8.10 Buffer Capacity

3. Buffer X is 0.20 M HA and 0.80 M NaA. Buffer Y is 0.80 M HA and 0.20 M NaA. Which statement is correct?

  1. AY has the higher pH and the greater capacity for added acid.
  2. BX has the higher pH and the greater capacity for added base.
  3. CX and Y have the same pH and the same capacity.
  4. DX has the higher pH and the greater capacity for added acid.

Answer: D. X has more base than acid, so its pH is above pKa. Added acid reacts with A⁻, and X has four times as much A⁻ as Y. So X can absorb more acid before failing.

Common trap: Choosing "same pH and capacity" because both total 1.0 M. The ratio and the separate amounts matter, not the total.

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