Quick answer: A Brønsted–Lowry acid donates a proton and a base accepts one. This Chemistry 11-12 test on acids pH buffers has 8 questions worth 20 marks, to do in 45 minutes: acids and bases, pH of nitric acid and pH of propanoic acid. The mark scheme is on a separate page; free printable PDF.
This test covers conjugate pairs, pH calculations, weak acids, buffers and a titration curve. You have 45 minutes and may use a calculator. Show all working and give each pH to two decimal places.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R3.1
Question 1 Acids and bases (3 marks)
Hydrogensulfate ions react with ammonia: HSO₄⁻ + NH₃ ⇌ SO₄²⁻ + NH₄⁺.- State what is meant by a Brønsted–Lowry acid. (1 mark)
- Identify the two conjugate acid–base pairs. (2 marks)
Question 2 pH of nitric acid (2 marks)
Calculate the pH of 0.0075 mol/dm³ nitric acid, a strong acid. Give your answer to 2 decimal places.Question 3 pH of propanoic acid (3 marks)
A pupil in Kampala has 0.200 mol/dm³ propanoic acid, Ka = 1.35 × 10⁻⁵ mol/dm³.- Write the expression for Ka. (1 mark)
- Calculate [H⁺] and the pH. (2 marks)
Question 4 Hydrocyanic acid buffer (2 marks)
A buffer contains 0.40 mol/dm³ hydrocyanic acid (pKa = 9.31) and 0.10 mol/dm³ sodium cyanide. Calculate its pH.Question 5 Buffer response to alkali (3 marks)
A buffer contains ethanoic acid and sodium ethanoate. Explain, with an equation, how it keeps the pH almost constant when a small amount of sodium hydroxide is added.Question 6 Titration of lactic acid (3 marks)
The graph shows the titration of 20.0 cm³ of lactic acid solution with 0.100 mol/dm³ NaOH.- Read the equivalence volume and calculate the concentration of the lactic acid. (1 mark)
- Read the pH at half-equivalence, state pKa and calculate Ka. (2 marks)
Question 7 pH at equivalence (2 marks)
Explain why the pH at the equivalence point of the titration in the previous question is greater than 7.Question 8 Strong against weak acid (2 marks)
0.10 mol/dm³ hydrochloric acid has pH 1.00 but 0.10 mol/dm³ ethanoic acid has pH 2.87. Explain the difference.
