Question

The radioactive isotope of lead, Pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. If 1 gram of this isotope is present initially, how long will it take for 70% of the lead to decay? (Round your answer to two decimal places.)

Answer 1

5.73 hours

Explanation:

70% decay means 30% remains

.3 = (1/2)^(t/3.3)

take the log of both sides

log.3 = t/3.3 * log1/2

Divide both sides by log 1/2

log.3 / log1/2 = t/3.3

use calculator

1.736965594166206 = t/3.3

multiply both sides by 3.3

5.73198646074848 = t

rounded

t = 5.73 hours