DP Chemistry (first assessment 2025)
Question 22M.1A.SL.TZ2.20
| Date | May 2022 | Marks available | [Maximum mark: 1] | Reference code | 22M.1A.SL.TZ2.20 |
| Level | SL | Paper | 1A | Time zone | TZ2 |
| Command term | Deduce | Question number | 20 | Adapted from | N/A |
20.
[Maximum mark: 1]
22M.1A.SL.TZ2.20
What is the strongest acid in the equation below?
H3AsO4 + H2O H2AsO4− + H3O+ Kc = 4.5 × 10−4
A. H3AsO4
B. H2O
C. H2AsO4−
D. H3O+
[1]
Markscheme
D
Examiners report
A challenging question that combined interpreting the implications of the value of the equilibrium constant with the Brønsted-Lowry concept. Most candidates focused on BL theory and the forward reaction only. 36% of the candidates answered the question correctly by selecting the acid involved in the backward reaction.
Syllabus sections
Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.3—The magnitude of the equilibrium constant indicates the extent of a reaction at equilibrium and is temperature dependent. Determine the relationships between K values for reactions that are the reverse of each other at the same temperature.