| Date | May 2015 | Marks available | 4 | Reference code | 15M.2.HL.TZ2.2 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 2 |
| Command term | State | Question number | 2 | Adapted from | N/A |
Question
Part 2 Nuclear energy and radioactivity
The graph shows the variation of binding energy per nucleon with nucleon number. The position for uranium-235 (U-235) is shown.
U-235 (23592U) can undergo alpha decay to form an isotope of thorium (Th).
(i) State the nuclear equation for this decay.
(ii) A sample of rock contains a mass of 5.6 mg of U-235 at the present day. The half-life of U-235 is 7.0×108 years. Determine the initial mass of the U-235 if the rock sample was formed 3.9×109 years ago.
Markscheme
(i) 23592U→23190Th+42α; (allow He for α; treat charge indications as neutral)
(ii) λ=1n27.0×108(=9.9×10−10year−1);
m0=5.6e3.9×109×9.9×10−10(mg)
266 mg; } (unit must match eg: allow 266 mg or 0.226 g but not 266 g or 0.266 kg)
or
number of half-lives =(3.9×1097×108=)5.57
initial mass = 5.6×25.57 ;
266 mg; } (unit must match eg: allow 266 mg or 0.226 g but not 266 g or 0.266 kg)
Award [3] for a bald correct answer.
