Question

A 200.0-ng sample of sodium-24 was stored in a lead-lined cabinet for 45 hours. The half-life of sodium-24 is 15 hours. How much sodium-24 remains?

Answer 1

75 ng
13 ng
100 ng
50 ng
25 ng

Answer 2

those are the answer choices

Answer 3

The half-life of the lead is 15 hours, which means that every 15 hours, half of the lead (from the start of the 15 hours) decays away.
We need to figure out how much lead is left after 45 hours, so we can use the information about the half-life and the amount of lead at the start.
At the start, there are 200.0 ng of lead. After 15 hours, this amount is cut in half. So, after the first 15 hours, there should be 100.0 ng of lead remaining.
After the next 15 hours, the amount is cut in half from 100.0 ng to... how much?
And then, after another 15 hours, the amount is cut in half from that amount to the final amount.
That's 45 total hours, which should give you your answer.