Question

a sample is in the second half - life. what percent of the sample is stable at this point? 12.5% 25% 50% 75%

Explanation:

Step1: Understand the concept of half - life

The half - life is the time required for half of a radioactive substance to decay.

Step2: Calculate the amount of sample remaining after the first half - life

Let the initial amount of the sample be \(N_0 = 100\%\). After the first half - life, the amount of the radioactive (unstable) sample remaining is \(N_1=\frac{N_0}{2}=50\%\), and the amount of stable sample is also \(50\%\) (since \(N_0 - N_1\)).

Step3: Calculate the amount of sample remaining after the second half - life

After the second half - life, the amount of the radioactive (unstable) sample remaining is \(N_2=\frac{N_1}{2}=\frac{50\%}{2} = 25\%\).
The amount of stable sample is \(N_0 - N_2\). Substituting \(N_0 = 100\%\) and \(N_2=25\%\), we get \(100\% - 25\%=75\%\)

Answer:

75%