OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 12
1 mark · Easy difficulty · Multiple Choice
Identify the conjugate acid-base pair from the given dissociation equation of the acid H3AsO4 in water.
Practise this questionQuestion
Question text
12 The equation shows the dissociation of the acid H3AsO4 in water.
H AsO + H O H AsO – + H O+
34 2 2 4 3
Which pair is a conjugate acid–base pair?
A H3AsO4 and H2O
B H AsO − and H O+
24 3
C H AsO and H O+
34 3
D H O+ and H O
Your answer [1]
Mark scheme
Show the mark scheme
12 D 1 AO1.2
How to answer it
Identifying Conjugate Acid-Base Pairs
What this question tests
This question assesses your understanding of Bronsted-Lowry acid-base theory, specifically the ability to identify species that differ by a single proton (H⁺ ion) on opposite sides of a chemical equilibrium.
Exam Breakdown
✅ Correct Answer
The correct option is D ( H₃O⁺ and H₂O ).
💡 Key Knowledge
- A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton ( H⁺ ).
- The acid has one more H⁺ than its conjugate base.
- In the given equation: H₃AsO₄ + H₂O ⇌ H₂AsO₄⁻ + H₃O⁺
🧠 Exam Technique
- Pair up species on the left with their reacting partners on the right based on structural similarity (arsenic species together, hydrogen/water species together).
- Check the proton difference: H₃O⁺ loses an H⁺ to become H₂O . Therefore, they form a valid conjugate pair.
❌ Common Errors
- Pairing reactants together (e.g., H₃AsO₄ and H₂O , option A) instead of matching a reactant with its corresponding product species.
- Confusing charge balances when removing or adding a proton.
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 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.