Question

70. The half-life of cobalt-60 is 5.3 years. What percent of a sample of cobal:-60 will remain after 21.2 years? A) 50% B) 40% C) 12.5% D) 10% E) 6.25%

Answer 1

Given that half life is 5.3 years i, e t_1/2 = 5.3years we know half life t_1/2 = 0.693/lambda lambda = Radioactive decay const. So, lambda = 0.693/t_1/2 = 0.693/5.3 year^-1 lambda = 0.13075 year^-1 \r\n Again if initial - 60 is N_0 and final - 60 is N then N = N_0 e^- lambda t t = 21.2 year. N = N_0 e^- (0.13075 times 21.2) N = 0.0625 N_0 So, % of remaining - 60 = 0.0625 times 100 = 6.25%