The activity of a radioactive sample falls from 640 counts per minute to 80 counts per minute in 15 days. What is the half-life?
Answer and explanation
Correct answer: B. 5 days
- Step 1: 640 → 320 → 160 → 80 is three halvings.
- Step 2: Three half-lives take 15 days, so one half-life = 15 ÷ 3 = 5 days.
Answer
5 days.
Why not the others: 3 days: this is the number of half-lives. 7.5 days: only one halving was assumed to take 15 days half. 15 days: the total time was used.
Practise more Chemistry in Society.
Free questions with an explanation for every one, and full timed National 5 (Scotland) mock tests.
More Chemistry in Society questions
- Zinc is added to copper sulfate solution. What do you expect to observe and why?
- In an electrochemical cell magnesium and copper electrodes are joined by wires. In which direction do electrons flow?
- What is the purpose of an ion bridge in an electrochemical cell?
- Magnesium atoms change to magnesium ions: Mg → Mg²⁺ + 2e⁻. Which statement is correct?
- Why does zinc coating protect an iron nail from rusting even when the coating is scratched?
- Which two substances are needed for iron to rust?