Question 22M.2.HL.TZ1.c
| Date | May 2022 | Marks available | [Maximum mark: 3] | Reference code | 22M.2.HL.TZ1.c |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Outline | Question number | c | Adapted from | N/A |
Potassium-40 decays by two processes.
The first process is that of beta-minus (β−) decay to form a calcium (Ca) nuclide.
Potassium-40 decays by a second process to argon-40. This decay accounts for 11 % of the total decay of the potassium-40.
Rocks can be dated by measuring the quantity of argon-40 gas trapped in them. One rock sample contains 340 µmol of potassium-40 and 12 µmol of argon-40.
Outline how the decay constant of potassium-40 was determined in the laboratory for a pure sample of the nuclide.
[3]
«use the mass of the sample to» determine number of potassium-40 atoms / nuclei in sample ✓
«use a counter to» determine (radio)activity / A of sample ✓
use A = λN «to determine the decay constant / λ» ✓
Describing methods of determining half-life continues to be a struggle for candidates with very few earning all three marks. Many candidates described a method more appropriate to measuring a short half- life, but even those descriptions fell far short of being acceptable.
