A thyroid cancer patient is given a dosage of 131I (half-life = 8.1 d). What fraction of the dosage of 131I will still be in the patient's thyroid after 72.9 days? (Let N0 and Nf represent the initial dose and the amount left after 72.9 days, respectively. Enter your answer as a fraction.)

Question

Nerdyhuman · Answer

@vocaloid

Vocaloid · Answer

On my phone, apologies for formatting

Nf = (N0)(1/2)^(t/half life)

If you divide both sides by N0, you’ll notice that you get Nf/N0 which is the fraction left, which is what you’re looking for. Right side is therefore (1/2)^(t/half life) where t is time (72.9 days) and half life is 8.1 days.

Nerdyhuman · Answer

It's 1/512

Nerdyhuman · Answer

What is the half-life of a radioactive isotope if the activity of a sample drops from 129,536 cpm to 253 cpm in 20.0 min?

Vocaloid · Answer

same equation, let Nf be 253 and N0 be 129536. since cpm = counts per minute, we're in minutes, so plug in t = 20.0 minutes, solve for half-life (your answer will also be in minutes)