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Date November 2019 Marks available 2 Reference code 19N.2.hl.TZ0.5
Level HL Paper 2 Time zone TZ0
Command term Apply and Describe Question number 5 Adapted from N/A

Question

Another common acid found in food is ethanoic acid.

A sample of ethanoic acid was titrated with sodium hydroxide solution, and the following pH curve obtained.

Annotate the graph to show the buffer region and the volume of sodium hydroxide at the equivalence point.

[2]
a.

Identify the most suitable indicator for the titration using section 22 of the data booklet.

[1]
b(i).

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]
b(ii).

Markscheme

buffer region on graph ✔
equivalence point/Veq on graph ✔

NOTE: Construction lines not required.

a.

phenolphthalein ✔

NOTE: Accept phenol red.

b(i).

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 ✔

b(ii).

Examiners report

[N/A]
a.
[N/A]
b(i).
[N/A]
b(ii).

Syllabus sections

Additional higher level (AHL) » Topic 18: Acids and bases » 18.3 pH curves
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Additional higher level (AHL) » Topic 18: Acids and bases
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