Iodine-131 is used to treat the thyroid…

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Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Iodine-131 is used to treat the thyroid. Its half-life is $8.02\ \text{d}$ , and its decay follows a first order law. A hospital receives a sample with an activity of $480\ \text{MBq}$ . What is the activity after $20\ \text{d}$ ? How long must the sample be kept for the activity to fall to $30\ \text{MBq}$ ?

Need a reminder? The key ideaShow

For a first order reaction, $t_{1/2} = \dfrac{0.693}{k}$ , and the fraction left after $n$ half-lives is $\left(\tfrac{1}{2}\right)^n$ . For a zero order reaction, $t_{1/2} = \dfrac{[R]_0}{2k}$ . In a pseudo first order reaction, $k' = k[\text{B}]_0$ .

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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