Question

What percentage of a radioactive substance remains after two half-lives?

Answer 1

25%

I know this because i have a doctorates in Percents

Answer 2

Answer: 25%.

Step-by-step explanation:

• The decay of a radioactive substance obeys the rules of first order reaction.

• In first order reaction half life time (t1/2) is constant and does not depend on the initial concentration of the substance (t1/2 = 0.693/k) where k is the rate constant.

• Half-life time (t1/2) is the time needed for the reaction that the initial concentration of the reactant (substance) reaches its half (reaction be completed by 50 %).

• radioactive substance (100%) (first half life) → 50 % (second half life) → 25 %

• Hence, the radioactive substance will reach 25 % of its concentration after two half-lives.