The questions are on the acids and bases pH test. Take it under real conditions before reading the mark scheme.
Quick answer: A strong acid ionises completely, so its pH comes straight from its concentration. This mark scheme gives the model answers and the marks for the 8 questions of the AP Chemistry test on acids and bases pH, from Brønsted–Lowry base to acidic and basic salts, with the equivalent answers to accept.
This is the acids and bases pH test answers for AP Chemistry, with the marks for each answer. First sit the test without notes, then mark it here and revise with the lesson.
Each model answer is followed by the marking guide. Award method marks even where the final value is wrong, and allow an error carried forward if later steps are consistent.
Level: AP Chemistry · Curriculum: AP Chem Unit 8
Acids and bases pH test answers: answers and marks
Question 1 Brønsted–Lowry base (2 marks)
- A proton acceptor.
- NH₄⁺.
Marking: 1 mark each.
Question 2 Strong acid pH (2 marks)
HI is strong, so [H₃O⁺] = 0.0042 mol/dm³. pH = −log₁₀(0.0042) = 2.38.Marking: 1 mark for recognising complete ionisation, 1 mark for the pH value.
Question 3 Weak acid calculation (3 marks)
Ka = x²/0.250, so x = √(4.5 × 10⁻⁶) = 2.121 × 10⁻³ mol/dm³. pH = −log₁₀(2.121 × 10⁻³) = 2.67.Marking: 1 mark for the Ka expression, 1 for x, 1 for the pH.
Question 4 Equal neutralisation (2 marks)
HCl ionises completely, HF only partly, so HCl has more H₃O⁺ and a lower pH. Both have the same amount of acid in moles, and the unionised HF keeps ionising as OH⁻ removes H₃O⁺, so both need the same NaOH volume.Marking: 1 mark for the pH explanation, 1 for the equal moles argument.
Question 5 Weak base calculation (3 marks)
- x = [OH⁻] = √(1.8 × 10⁻⁵ × 0.150) = 1.643 × 10⁻³ mol/dm³; pOH = 2.78; pH = 11.22.
- Ka = 1.0 × 10⁻¹⁴ / 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰.
Marking: (a) 1 mark for [OH⁻], 1 for pH; (b) 1 mark.
Question 6 Comparing pH values (2 marks)
[H₃O⁺] in A = 10⁻³ and in B = 10⁻⁵ mol/dm³, so the ratio is 100. A is more acidic.Marking: 1 mark for the ratio, 1 for the conclusion.
Question 7 Ranking three acids (3 marks)
- Lowest pH means most H₃O⁺ at equal concentration: P, then Q, then R.
- [H₃O⁺] = 10⁻²·²⁰ = 6.31 × 10⁻³ mol/dm³; Ka = (6.31 × 10⁻³)² / (0.100 − 6.31 × 10⁻³) = 4.2 × 10⁻⁴.
Marking: (a) 1 mark; (b) 1 mark for [H₃O⁺], 1 for Ka.
Question 8 Acidic and basic salts (3 marks)
NH₄⁺ is the conjugate acid of a weak base: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, so the solution is acidic. CH₃COO⁻ is the conjugate base of a weak acid: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, so the solution is basic. Cl⁻ and Na⁺ do not react with water.Marking: 1 mark per equation, 1 mark for stating that Na⁺ and Cl⁻ are spectators.
