OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2022: Question 12

1 mark · Medium difficulty · Multiple Choice

Identify the Brønsted-Lowry acids in the equilibrium mixture of ammonia and water.

Practise this question

Question

Multiple choice question 12 asking to identify the Brønsted-Lowry acids in the equilibrium mixture of ammonia and water, with four options: A (H2O and OH-), B (OH- and NH3), C (NH4+ and H2O), and D (NH4+ and NH3).
Question text

12 Ammonia and water react to set up an acid–base equilibrium.

What are the Brønsted–Lowry acids in the equilibrium mixture?

A H O and OH–

B OH– and NH

C NH + and H O

D NH + and NH

Your answer [1]

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer is C.

12 C 1 1.2

How to answer it

Identifying Brönsted-Lowry Acids in Equilibrium

What this question tests

This question assesses your understanding of the Brönsted-Lowry theory of acids and bases, specifically the ability to identify proton (H⁺) donors in both the forward and reverse directions of an equilibrium mixture.

Question 12

Identifying Brönsted-Lowry Acids

✅ Correct Answer: C

The correct option is C ( NH₄⁺ and H₂O ).

💡 Key Knowledge

  • Brönsted-Lowry Acid: A proton ( H⁺ ) donor.
  • The Equilibrium: NH₃ (aq) + H₂O (l) ⇌ NH₄⁺ (aq) + OH⁻ (aq)
  • To find the acids, look at both sides of the equation to see which species give away a proton to form their conjugate bases.

🧠 Exam Technique

  • Write out the full balanced equilibrium equation first to visualise proton transfer clearly.
  • Trace the pairs: H₂O donates a H⁺ to become OH⁻ (making H₂O an acid).
  • NH₄⁺ can donate a H⁺ to become NH₃ (making NH₄⁺ an acid in the reverse reaction).

❌ Common Errors

  • Mistaking NH₃ for an acid (it acts as a Brönsted-Lowry base by accepting a proton to form NH₄⁺ ).
  • Confusing conjugate acid-base pairs with the species present in the equilibrium mixture.
Mark Scheme Breakdown: 1 mark awarded for selecting option C.

Topics

Module 5: Physical chemistry and transition elements · 5.1 Rates, equilibrium and pH

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.