DP Chemistry (first assessment 2025)
Question 19N.2.HL.TZ0.5b(ii)
| Date | November 2019 | Marks available | [Maximum mark: 2] | Reference code | 19N.2.HL.TZ0.5b(ii) |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Apply, Describe | Question number | b(ii) | Adapted from | N/A |
b(ii).
[Maximum mark: 2]
19N.2.HL.TZ0.5b(ii)
(b(ii))
Describe, using a suitable equation, how the buffer solution formed during the titration resists pH changes when a small amount of acid is added.
[2]
Markscheme
ALTERNATIVE 1:
H+ (aq) + CH3COO– (aq) → CH3COOH (aq) ✔
added acid neutralised by ethanoate ions
OR
«weak» CH3COOH (aq)/ethanoic acid replaces H+ (aq)
OR
CH3COOH/CH3COO– ratio virtually/mostly unchanged ✔
ALTERNATIVE 2:
CH3COOH (aq) H+ (aq) + CH3COO– (aq) ✔
equilibrium shifts to the ethanoic acid side
OR
CH3COOH/CH3COO− ratio virtually/mostly unchanged ✔
Syllabus sections
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.16—A buffer solution is one that resists change in pH on the addition of small amounts of acid or alkali. Describe the composition of acidic and basic buffers and explain their actions.