Question

A certain radioactive element has a half life of 6749 years. How much of a 7.87 g sample is left after 16767 years?

Answer 1

half - life = 6749 years

decay constant k = 0.693 / half - life

                             = 0.693 / 6749

                             = 1.027 x 10^-4 yr-1

mass of sample = Ao = 7.87 g

time = 16767 years

k = 1/t ln (Ao / At)

1.027 x 10^-4 = 1 / 16767 ln (7.87 / At)

At = 1.407 g

sample is left over = 1.41 g