DP Chemistry (first assessment 2025)

Question 19N.2.HL.TZ0.5

Date	November 2019	Marks available	[Maximum mark: 5]	Reference code	19N.2.HL.TZ0.5
Level	HL	Paper	2	Time zone	TZ0
Command term	Annotate, Apply, Describe, Identify, Show	Question number	5	Adapted from	N/A

[Maximum mark: 5]

19N.2.HL.TZ0.5

Another common acid found in food is ethanoic acid.

(a)

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]

Markscheme

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

NOTE: Construction lines not required.

(b(i))

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

[1]

Markscheme

phenolphthalein ✔

NOTE: Accept phenol red.

(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) $\rightleftharpoons$ H⁺ (aq) + CH₃COO^– (aq) ✔

equilibrium shifts to the ethanoic acid side
OR
CH₃COOH/CH₃COO⁻ ratio virtually/mostly unchanged ✔

Syllabus sections