90 Sr Half life = 28.1 years
What is the initial mass of 90 Sr if after 200 years, 1.00 grams is left undecayed?
Expert Answer
For a first order reaction rate constant , k = ( 2.303 /t )x log ( Mo / M)
Where
Mo = initial mass = ?
M= Mass left after time t = 1.00 g
K = rate constant = 0.693 /t1/2 = 0.693 / 28.1 = 0.025 year^-1
t = time = 200 years
Plug the values we get log ( Mo / M) = (kt) / 2.303
= 2.142
Mo / M = 102.142 = 138.6
Mo = 138.6×1.00 = 138.6 g