Question

Chromium-51 is a radioactive isotope used to label red blood cells to identify if they remain intact in the body. It has a half-life of 27.7 days. If 0.25 mg is an approximate dose amount, how much remains after 83.1 days?

Answer 1

Since the half life is the the time taken for half of the Chromium-51 to decay, for n half lives, the mass of Chromium-51 will fall to 1/(2^n) of its original value. 83.1/27.7 = 3 so the mass of Chromium-51 will fall by a factor of 1/(2^3) = 1/8.
0.25 mg / 8 = 0.031 mg (2 significant figures).