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Date	November 2020	Marks available	1	Reference code	20N.2.hl.TZ0.5
Level	HL	Paper	2	Time zone	TZ0
Command term	Justify and Suggest	Question number	5	Adapted from	N/A

Question

A student performs a titration to determine the concentration of ethanoic acid, ${CH}_{3} COOH$ , in vinegar using potassium hydroxide.

The pH curve for the reaction is given.

Write a balanced equation for the reaction.

[1]

Identify the major species, other than water and potassium ions, at these points.

[2]

b(i).

State a suitable indicator for this titration. Use section 22 of the data booklet

[1]

b(ii).

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

[1]

b(iii).

State the $K_{a}$ expression for ethanoic acid.

[1]

Calculate the $K_{b}$ of the conjugate base of ethanoic acid using sections 2 and 21 of the data booklet.

[1]

In a titration, $25.00 {cm}^{3}$ of vinegar required $20.75 {cm}^{3}$ of $1.00 mol {dm}^{- 3}$ potassium hydroxide to reach the end-point.

Calculate the concentration of ethanoic acid in the vinegar.

[2]

Potassium hydroxide solutions can react with carbon dioxide from the air. The solution was made one day prior to using it in the titration.

State the type of error that would result from the student’s approach.

[1]

f(i).

Potassium hydroxide solutions can react with carbon dioxide from the air. The solution was made one day prior to using it in the titration.

Predict, giving a reason, the effect of this error on the calculated concentration of ethanoic acid in 5(e).

[2]

f(ii).

Markscheme

${CH}_{3} COOH (aq) + KOH (aq) \to {CH}_{3} COOK (aq) + H_{2} O (l)$ ✔

Accept the ionic equation.

B: ${CH}_{3} COOH$ AND ${CH}_{3} {COO}^{-}$ ✔

C: ${CH}_{3} {COO}^{-}$ ✔

Accept names.

Accept $C H_{3} C O O K$ for $C H_{3} C O O^{-}$

b(i).

phenolphthalein ✔

Accept “phenol red” or “bromothymol blue”.

b(ii).

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/ ${CH}_{3} COOH$ /ethanoic acid AND conjugate base/ ${CH}_{3} {COO}^{-}$ /ethanoate ✔

b(iii).

$K_{a} = \frac{[{CH}_{3} {COO}^{-}] [H_{3} O^{+}]}{[{CH}_{3} COOH]}$

Accept $H^{+}$ instead of $H_{3} O^{+}$ .

$« K_{a} = 10^{- 4.76} = 1.7 \times 10^{- 5} »$
$« K_{w} = K_{a} \cdot K_{b} = 1.0 \times 10^{- 14} = 1.7 \times 10^{- 5} \times K_{b} »$
$« K_{b} = » 5.8 \times 10^{- 10}$ ✔

Accept answers between $5.7 - 5.9 \times 10^{- 10}$ .

$« n (KOH) = 0.02075 {dm}^{3} \times 1.00 mol {dm}^{- 3} = » 0.0208 « mol »$ ✔

$« n (KOH) = n ({CH}_{3} COOH) »$
$« [{CH}_{3} COOH] = \frac{0.0208 mol}{0.02500 {dm}^{3}} = » 0.830 « mol {dm}^{- 3} »$ ✔

Award [2] for correct final answer.

systematic «error» ✔

f(i).

$[{CH}_{3} COOH]$ would be higher ✔

actual $[KOH]$ is lower «than the value in calculation»
OR
larger volume of $KOH$ «solution» needed to neutralize the acid ✔

Accept $K O H$ partially neutralised by $C O_{2}$ from air.

f(ii).

Examiners report

Most candidates could write a balanced neutralization equation.

Identifying species present at various points along a pH titration curve was one of the most poorly answered questions in the exam. Very few candidates realized there were two major species at point B even when they were able in general to realize that B was a buffer zone.

b(i).

Almost all candidates could identify a suitable indicator to use in a titration of a weak acid with a strong base.

b(ii).

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

b(iii).

Poorly answered with only 50% correctly writing a K_a expression. The major error was in candidates trying to calculate a K_a rather than write an expression for it.

[N/A]

Like with other calculations in this exam, the majority of candidates could correctly determine a concentration from titration data.

80% of candidates could identify the method used as a systematic error, with some stating human or random error.

f(i).

Most candidates identified that the systematic error would result in the concentration of the alkali being lowered but then failed to propagate this through to the effect on the concentration of the acid.

f(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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Markscheme

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