Approximately what mass of 90sr (ti / 2 = 28.8 yr.) has the same activity as 1 g of 60co (ti / 2 = 5.26

Activity (A) of an element is related to the number of atoms of that element N as follows:

A = k*N (1)

k = rate constant = 0.693/t1/2(2)

N = # moles of that element * Avogadro's number Na

For Sr: t1/2 = 28.8 yr

A(Sr) = 0.693/28.8 * N(Sr) = 0.0241 * N(Sr)

A(Sr) = 0.0241*(mass Sr/90)*Na= 0.000268 *mass Sr *Na (3)

For Co : t1/2 = 5.26 yr

A(Co) = 0.693/5.26 *N(Co) = 0.1317 *N(Co)

A(Co) = 0.1317 * (1/60)*Na = 0.00219 * Na (4)

It is given that:

A(Sr) = A(Co)

0.000268 *mass Sr *Na = 0.00219 * Na

mass Sr = 0.00219/0.000268 = 8.17 g

h let me think

it's all four options.