DP Chemistry (first assessment 2025)
Question 19M.2.HL.TZ2.5f
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.2.HL.TZ2.5f |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Sketch | Question number | f | Adapted from | N/A |
f.
[Maximum mark: 2]
19M.2.HL.TZ2.5f
(f)
Aqueous sodium hydrogencarbonate has a pH of approximately 7 at 298 K.
Sketch a graph of pH against volume when 25.0cm3 of 0.100 mol dm−3 NaOH (aq) is gradually added to 10.0cm3 of 0.0500 mol dm−3 NaHCO3 (aq).
[2]
Markscheme
S-shaped curve from ~7 to between 12 and 14 [✔]
equivalence point at 5 cm3 [✔]
Note: Accept starting point >6~7.
Examiners report
Most students that got 1mark for this titration curve was for the general shape, because few realized they had the data to calculate the equivalence point. There were also some difficulties in establishing the starting point even if it was specified in the stem.
Syllabus sections
Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.13—pH curves of different combinations of strong and weak monoprotic acids and bases have characteristic shapes and features. Interpret the general shapes of pH curves for all four combinations of strong and weak acids and bases.