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Date November 2020 Marks available 1 Reference code 20N.3.sl.TZ0.10
Level SL Paper 3 Time zone TZ0
Command term Deduce Question number 10 Adapted from N/A

Question

1.57% of the mass of a rock weighing 46.5kg is uranium(IV) oxide, UO299.28% of the uranium atoms in the rock are uranium-238, U238.

Show that the mass of the U238 isotope in the rock is 0.639kg.

[2]
a.

The half-life of 238U is 4.46×109 years. Calculate the mass of 238that remains after 0.639kg has decayed for 2.23×1010 years.

[2]
b.

Outline a health risk produced by exposure to radioactive decay.

[1]
c.

Deduce the nuclear equation for the decay of uranium-238 to thorium-234.

[1]
d.

Thorium-234 has a higher binding energy per nucleon than uranium-238. Outline what is meant by the binding energy of a nucleus.

[1]
e.

Markscheme

«mass%fraction of U in UO2=»238.03238.03+2×16 /0.881/88.1% ✔

46.5«kg»×0.0157×0.881×0.9928«=0.639kg» ✔

Award [1 max] for omitting mass composition (giving 0.725kg).

M2 is for numerical setup, not for final value of 0.639kg.

a.

Alternative 1
«2.23×1010 year4.46×109 year=»5.00«half-lives» ✔

«m=0.639kg×(0.5)5=»0.0200«kg» ✔


Alternative 2
«𝜆=ln24.46×109 year=»1.554×1010«year1» ✔

«m=0.639kg×𝑒1.554×1010 year1×2.23×1010 year=»0.0200«kg» ✔


Award [2] for correct final answer.

b.

Any one:

«genetic» mutations ✔

«could cause» cancer ✔
Accept specific named types of cancer.

cells «in body» altered ✔

cells «in body» cannot function ✔

damaged DNA/proteins/enzymes/organs/tissue ✔

«radiation» burns ✔

hair loss ✔

damage in foetuses ✔

damages/weakens immune system ✔

 

c.

U92238Th90234+He24 ✔

Do not penalize missing atomic numbers in the equation.

Accept “α” for "He”.

d.

energy required to separate a nucleus into protons and neutrons/nucleons
OR
energy released when nucleus was formed from «individual/free/isolated» protons and neutrons/nucleons ✔


Do not accept “energy released when atom was formed”.

e.

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.

a.

This continues to be a topic that students understand well, and probably more used alternative 1 to calculate the half-life. 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.

b.

A well answered question.

c.

Only very weak candidates lost this mark.

d.

Many students appeared to have portion of the answer but not the entire concept. They seemed to have studied previous MS by heart and entered an answer that was mostly correct but didn't address the question. It is important that student understand the material and not try to memorize their way through the topics.

e.

Syllabus sections

Options » C: Energy » C.3 Nuclear fusion and fission
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Options » C: Energy
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