Answer:
12.5mg of technetium-99 remain
Explanation:
When a radioactive isotope decays in the time, its concentration follows the equation:
ln[A] = -kt + ln[A]₀
Where [A] could be taken as the mass of radioactive isotope after time t (18h),
k is rate constant = ln 2 / Half-Life = ln 2 / 6hours = 0.1155hours⁻¹
[A]₀ is initial amount of the isotope = 100.0mg
Replacing:
ln[A] = -0.1155hours⁻¹ *18hours+ ln[100.0mg]
ln[A] = 2.5257
[A] =
