Question

A radioactive substance has half life of 3 hours. 75 % of the substance would decay in ____________.

Options

• 6 hours

• 12 hours

• 9 hours

• 3 hours

Answer 1

A radioactive substance has half life of 3 hours. 75 % of the substance would decay in 6 hours.

Explanation:

Half life T = 3 hours.

When 75% of the substance decays, 25% remains

${\displaystyle \therefore \frac{\text{N}}{{\text{N}}_0}=\frac{1}{4}=\frac{1}{2^{\text{n}}}}$

where n is the number of half live.

∴ n = 2

∴ time t = 2 x 3 = 6 hours