A procedure requires use of 140mg of technetium-99m has a half life of 6.0 hours. How many mill. after 18 h.

Question

A medical procedure requires use of 140mg of technetium-99m which has a half life of 6.0 hours. How many milligrams remain after 18 hours? Can you show your work??

Answer 1

Total dose of Technetium-99m = 140 mg

Amount at the end of 1st half-life (6hrs) = 140/2 = 70mg

Amount at the end of 2nd half-life (12hrs) = 70/2 = 35 mg

Amount remaining at the end of 3rd half life (18hrs) = 35/2 = 17.5 mg

Generalization for

Amount of drug remaining at the end of N half lives = X / 2^N

where X is the initial amount & N= No. ofhalf lives

So, in this example, X= 140 mg & N = 3

Amount = 140/ 2^3 = 140/8 = 17.5 mg