DP Chemistry (last assessment 2024)

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Question 19M.2.hl.TZ2.f

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Date May 2019 Marks available [Maximum mark: 2] Reference code 19M.2.hl.TZ2.f
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.f

Carbonated water is produced when carbon dioxide is dissolved in water under pressure. The following equilibria are established.

Equilibrium (1)  CO2 (g)  CO2 (aq)

Equilibrium (2)  CO2 (aq) + H2O (l) H+ (aq) + HCO3− (aq)

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.

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