Answer:
3 half-lives.
Explanation:
- The decay of radioactive elements obeys first-order kinetics.
- Half-life time is the time required for the initial concentration be decreased to its half value.
Half-life time is constant for first-order reaction.
21% of the sample (10 g) represents (10.0 x 0.21 = 2.1 g).
10 g (first half-life) → 5 g (second half-life) → 2.5 g (third half-life) → 1.25 g.
∴ 3 half-lives are needed for the sample to reach less than 21% (<2.1 g) of its initial value.
