Question 17.4: Half-Life How long does it take for a 1.80-mol sample of Th-......
Half-Life
How long does it take for a 1.80-mol sample of Th-228 (which has a half-life of 1.9 years) to decay to 0.225 mol?
Step-by-Step
Draw a table showing the amount of Th-228 as a function of number of halflives. For each half-life, divide the amount of Th-228 by 2.
It takes three half-lives or 5.7 years for the sample to decay to 0.225 mol.
| Amount of Th-228 | Number of Half-Lives | Time (yrs) |
| 1.80 mol | 0 | 0 |
| 0.900 mol | 1 | 1.9 |
| 0.450 mol | 2 | 3.8 |
| 0.225 mol | 3 | 5.7 |
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