Quick answer: A buffer is a solution of a weak acid and its conjugate base that resists pH change. This AP Chemistry worksheet on buffer solutions gives 20 problems from easy to hard: equal amounts of acid and base, unequal buffer ratio and spot the buffers. The worked answers are on a separate page, and the worksheet is a free printable PDF.
These problems cover Unit 8 of AP Chemistry: buffers, the Henderson–Hasselbalch equation, buffer capacity, titration curves and indicators. They are also suitable for A Level and first-year university students.
Level 1 problems use the equation directly. Level 2 problems add mole tables for added acid or base and the pH of a solution at several points on a curve. The last five problems are level 3 and combine several steps or ask for derivations and explanations.
Level: AP Chemistry · Curriculum: AP Chem Unit 8
Problem 1 Equal amounts of acid and base ★★★
A buffer contains 0.25 mol/dm³ CH₃COOH and 0.25 mol/dm³ CH₃COONa (pKa = 4.74). Calculate its pH.Problem 2 Unequal buffer ratio ★★★
Chiamaka in Lagos prepares a buffer with 0.40 mol/dm³ ethanoate and 0.20 mol/dm³ ethanoic acid (pKa = 4.74). Calculate its pH.Problem 3 Spot the buffers ★★★
Decide which mixtures are buffers: (a) HCl and NaCl, (b) HF and NaF, (c) NH₃ and NH₄Cl, (d) NaCl and NaOH.Problem 4 Choose a buffer pair ★★★
A lab needs a buffer at pH 9.2. The options are CH₃COOH/CH₃COO⁻ (pKa 4.74) and NH₄⁺/NH₃ (pKa 9.25). Choose and justify.Problem 5 Equivalence volume ★★★
Emma in Leeds titrates 25.0 cm³ of 0.100 mol/dm³ HCl with 0.0500 mol/dm³ NaOH. Calculate the volume of NaOH needed to reach equivalence.Problem 6 pKa from half-equivalence ★★★
In the titration of a weak monoprotic acid, the pH is 4.76 when exactly half of the equivalence volume has been added.1. State pKa of the acid.
2. Calculate Ka.
Problem 7 Ammonia buffer ★★★
Diego in Texas mixes 0.30 mol/dm³ NH₃ with 0.20 mol/dm³ NH₄Cl. The pKa of NH₄⁺ is 9.25. Calculate the pH.Problem 8 Adding base to a buffer ★★★
A 100.0 cm³ buffer contains 0.0200 mol CH₃COOH and 0.0200 mol CH₃COO⁻ (pKa 4.74). Then 5.0 cm³ of 1.0 mol/dm³ NaOH is added.1. Calculate the amount of NaOH added.
2. Find the new amounts of HA and A⁻.
3. Calculate the new pH.
Problem 9 Adding acid to a buffer ★★★
Wanjiru in Nairobi has 250 cm³ of buffer containing 0.025 mol NH₃ and 0.025 mol NH₄⁺ (pKa of NH₄⁺ = 9.25). She adds 0.0050 mol HCl.1. Write the equation for the reaction of the added acid.
2. Calculate the new amounts of NH₃ and NH₄⁺.
3. Calculate the new pH.
Problem 10 Making a buffer of given pH ★★★
Bilal in Lahore wants a buffer of pH 4.90. He has 0.200 mol of ethanoic acid (pKa 4.74) in solution. M(CH₃COONa) = 82.0 g/mol.1. Calculate the required ratio [A⁻]/[HA].
2. Calculate the amount of sodium ethanoate needed.
3. Calculate its mass.
Problem 11 Partial neutralisation ★★★
Mei in Singapore adds 20.0 cm³ of 0.100 mol/dm³ NaOH to 50.0 cm³ of 0.200 mol/dm³ CH₃COOH.2. Find the amounts of HA and A⁻ after reaction.
3. Calculate the pH (pKa 4.74).
Problem 12 Before equivalence, strong acid ★★★
In a titration 20.0 cm³ of 0.100 mol/dm³ NaOH has been added to 25.0 cm³ of 0.100 mol/dm³ HCl. Calculate the pH.Problem 13 Equivalence of ethanoic acid ★★★
Calculate the pH at equivalence when 25.0 cm³ of 0.100 mol/dm³ CH₃COOH is titrated with 0.100 mol/dm³ NaOH (Ka = 1.8 × 10⁻⁵).1. Find [CH₃COO⁻] at equivalence.
2. Calculate Kb.
3. Calculate the pH.
Problem 14 Choose the indicator ★★★
Indicator ranges: methyl orange 3.1–4.4, bromothymol blue 6.0–7.6, phenolphthalein 8.3–10.0. Choose an indicator for (a) HCl with NaOH, (b) CH₃COOH with NaOH.Problem 15 Comparing capacity ★★★
Two 1.00 dm³ buffers use CH₃COOH/CH₃COO⁻ (pKa 4.74). Buffer A has 0.50 mol of each; buffer B has 0.050 mol of each. Kofi adds 0.020 mol HCl to each.2. Calculate the new pH of B.
3. Explain which has the greater capacity.
Problem 16 Complete titration of methanoic acid ★★★
Aarav in Pune titrates 25.0 cm³ of 0.100 mol/dm³ HCOOH (Ka = 1.8 × 10⁻⁴) with 0.100 mol/dm³ NaOH. Calculate the pH after adding (a) 0 cm³, (b) 12.5 cm³, (c) 25.0 cm³, (d) 30.0 cm³.Problem 17 Derive the buffer equation ★★★
Starting from Ka = [H₃O⁺][A⁻]/[HA], derive the Henderson–Hasselbalch equation and calculate pH when [A⁻]/[HA] = 0.10 for an acid with pKa 4.74.Problem 18 Buffer from ammonia and HCl ★★★
Mei in Singapore needs a buffer of pH 9.00. She has 100.0 cm³ of 0.50 mol/dm³ NH₃ and 0.50 mol/dm³ HCl (pKa of NH₄⁺ = 9.25).1. Calculate the required ratio n(NH₃)/n(NH₄⁺).
2. Calculate the amount of HCl to add.
3. Calculate its volume.
Problem 19 Unknown weak acid ★★★
Diego in Texas titrates 25.0 cm³ of a monoprotic weak acid with 0.100 mol/dm³ NaOH. Equivalence is reached at 31.5 cm³, and the pH after adding 15.75 cm³ is 4.20.1. Calculate the concentration of the acid.
2. State pKa and calculate Ka.
3. Explain how you know pH = pKa at 15.75 cm³.
Problem 20 Capacity limit ★★★
A 1.00 dm³ buffer contains 0.30 mol CH₃COOH and 0.30 mol CH₃COO⁻ (pKa 4.74). Wanjiru will accept a drop of at most 0.10 pH unit.1. Write the condition on the new ratio [A⁻]/[HA].
2. Calculate the maximum amount of HCl that can be added.
3. Calculate the volume of 1.0 mol/dm³ HCl this represents.
