Quick answer: A buffer is a solution of a weak acid and its conjugate base that resists pH change. This AP Chemistry test on buffer solutions has 8 questions worth 20 marks, to do in 45 minutes: defining a buffer, buffer pH and adding acid. The mark scheme is on a separate page; free printable PDF.
This AP Chemistry test on buffer solutions test works like a real class test. The mark scheme is on a separate page, so revise with the lesson first.
Answer all questions. Show your working. A calculator is allowed. Use Kw = 1.0 × 10⁻¹⁴ and pKa(CH₃COOH) = 4.74 unless the question gives other data.
Level: AP Chemistry · Curriculum: AP Chem Unit 8
Buffer solutions test: the test questions
Question 1 Defining a buffer (2 marks)
Define a buffer solution and give one example of a suitable pair of substances. (2 marks)Question 2 Buffer pH (3 marks)
A buffer contains 0.15 mol/dm³ HF and 0.25 mol/dm³ NaF. pKa of HF = 3.17. Calculate its pH. (3 marks)Question 3 Adding acid (3 marks)
A 500 cm³ buffer contains 0.15 mol NH₃ and 0.20 mol NH₄Cl (pKa of NH₄⁺ = 9.25). Calculate the pH after 0.020 mol HCl is added. (3 marks)Question 4 Effective range (2 marks)
Explain why a buffer works best when pH is within one unit of pKa. (2 marks)Question 5 Reading a titration curve (3 marks)
- State the equivalence volume. (1 mark)
- Use the curve to state pKa. (1 mark)
- Choose from methyl orange (3.1–4.4), bromothymol blue (6.0–7.6) and phenolphthalein (8.3–10.0) the best indicator. (1 mark)
Question 6 Strong acid titration (2 marks)
Calculate the volume of 0.100 mol/dm³ NaOH needed to neutralise 20.0 cm³ of 0.150 mol/dm³ HCl. (2 marks)Question 7 Equivalence pH of HCN (3 marks)
25.0 cm³ of 0.200 mol/dm³ HCN (Ka = 4.9 × 10⁻¹⁰) is titrated with 0.200 mol/dm³ NaOH. Calculate the pH at equivalence and explain why it is above 7. (3 marks)Question 8 Not a buffer (2 marks)
Explain why a mixture of HCl and NaCl cannot act as a buffer. (2 marks)
