DP Chemistry (last assessment 2024)

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Question 20N.2.hl.TZ0.b(iii)

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Date November 2020 Marks available [Maximum mark: 1] Reference code 20N.2.hl.TZ0.b(iii)
Level hl Paper 2 Time zone TZ0
Command term Justify, Suggest Question number b(iii) Adapted from N/A
b(iii).
[Maximum mark: 1]
20N.2.hl.TZ0.b(iii)

A student performs a titration to determine the concentration of ethanoic acid, CH3COOH, in vinegar using potassium hydroxide.

The pH curve for the reaction is given.

Suggest, giving a reason, which point on the curve is considered a buffer region.

[1]

Markscheme

B AND the region where small additions «of the base/KOH » result in little or no
change in pH
OR
B AND the flattest region of the curve «at intermediate pH/before equivalence
point »
OR
B AND half the volume needed to reach equivalence point
OR
B AND similar amounts of weak acid/CH3COOH/ethanoic acid AND conjugate base/CH3COO−/ethanoate ✔

Examiners report

Most students could identify a buffer zone region in a titration but very few (50%) could coherently explain why.

Syllabus sections