DP Chemistry (last assessment 2024)

Question 20N.3.sl.TZ0.a

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

[Maximum mark: 2]

20N.3.sl.TZ0.a

$1.57 %$ of the mass of a rock weighing $46.5 kg$ is uranium(IV) oxide, ${UO}_{2}$ . $99.28 %$ of the uranium atoms in the rock are uranium-238, ${}^{238}U$ .

Show that the mass of the ${}_{238}U$ isotope in the rock is $0.639 kg$ .

[2]

Markscheme

$« \frac{mass %}{fraction of U in {UO}_{2}} = » \frac{238.03}{238.03 + 2 \times 16}$ / $0.881$ / $88.1 %$ ✔

$46.5 « kg » \times 0.0157 \times 0.881 \times 0.9928 « = 0.639 kg »$ ✔

Award [1 max] for omitting mass composition (giving $0.725 k g$ ).

M2 is for numerical setup, not for final value of $0.639 k g$ .

Examiners report

This was a very different question as student were given the answer and it was the work that was being marked. Students should always clearly show their calculations so examiners can award marks throughout the question and potentially award ECF if possible. It is very difficult to do this when students do not show work clearly.

Syllabus sections

Core

Core » Topic 1: Stoichiometric relationships » 1.3 Reacting masses and volumes

Core » Topic 1: Stoichiometric relationships

Question 20N.3.sl.TZ0.a

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