DP Chemistry (last assessment 2024)

Question 20N.3.sl.TZ0.2e

Date	November 2020	Marks available	[Maximum mark: 2]	Reference code	20N.3.sl.TZ0.2e
Level	sl	Paper	3	Time zone	TZ0
Command term	Calculate	Question number	e	Adapted from	N/A

[Maximum mark: 2]

20N.3.sl.TZ0.2e

An investigation was carried out to determine the effect of chain length of the alcohol on the equilibrium constant, $K_{c}$ , for the reversible reaction:

$ROH + {CH}_{3} COOH \overset{H^{+} (aq)}{⇌} {CH}_{3} COOR + H_{2} O$

The reactants, products and the catalyst form a homogeneous mixture.

Fixed volumes of each alcohol, the ethanoic acid and the sulfuric acid catalyst were placed in sealed conical flasks.

At equilibrium, the flasks were placed in an ice bath, and samples of each flask titrated with $NaOH (aq)$ to determine the ethanoic acid concentration present in the equilibrium mixture.

The following processed results were obtained.

(e)

Calculate the percentage uncertainty and percentage error in the experimentally determined value of $K_{c}$ for methanol.

[2]

Markscheme

Percentage uncertainty:
$« \frac{0.4 \times 100}{6.5} = » 6 « % »$ ✔

Percentage error:
$« \frac{6.5 - 5.3}{5.3} = » 23 « % »$ ✔

Award [1 max] if calculations are reversed OR if incorrect alcohol is used.

Examiners report

Many students scored both points and others at least one. Weaker students inverted the calculations.

Syllabus sections

Core

Core » Topic 11: Measurement and data processing » 11.1 Uncertainties and errors in measurement and results

Core » Topic 11: Measurement and data processing

Question 20N.3.sl.TZ0.2e

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