Question

Phosphorus-33 is a radioisotope with a half-life of 15 days. If you start with 20 g of phosphorus-33:

a. How many half-lives will have occurred after 45 days?
b. How many grams will remain after 45 days?
a) 3; 2.5 g
b) 2; 5 g
c) 4; 2.5 g
d) 5; 5 g

Answer 1

After 45 days, or 3 half-lives, 2.5 grams of the original 20 grams of phosphorus-33 will remain, given that the half-life of phosphorus-33 is 15 days.

Step-by-step explanation:

The half-life of a radioisotope is the time it takes for half of the original amount to radioactively decay. For phosphorus-33 with a half-life of 15 days:

1. After 45 days, meaning after 3 half-lives (15 days × 3), we can determine the remaining amount.
2. Starting with 20 g of phosphorus-33, it will halve three times: 20 g to 10 g to 5 g to 2.5 g.

Therefore, the correct answer is:

• a) 3 half-lives will have occurred after 45 days.
• b) 2.5 grams of phosphorus-33 will remain after 45 days.