Hello, I'm new to the forum, nice to meet you all ^^

My question:

the Isotop Sr^{90} has a half life of 28.1 years. how much would remain from a 1g of Sr after 25 years.

I toke the topic of half-life in geology and general chem before, but i dont have my textbook at the moment to review and find the answer ><

By dividing 28.1/25, this would give us the number of half lifes in 28.1 years i think?

But i dont know how to take it from there, and how to find the mass after 28.1 years.

A 2nd part of the question asks about the percentage of the initial acivities remains after 25 years. i found the decay constant, k in 1/sec., but couldnt continou.

I would ask the instractor if i could, but am taking this course as an independent study. sorry if my question was not very clear, english is my 2nd language.

Thanks alot and i hope to hear from you soon.

the formula for radioactive decay is:

N(t) = N(0) * (1/2)^(t/tau) with N = numbers of cores and tau = half life time.

so 1g Sr = 0,0114129194 mol = 6,8706 x 10^21 cores.

so: N(25 years) = 6,8706 x 10^21 * (1/2)^25/28,1

--> N(25 years) = 3,7083 x 10^21 cores

--> that's 0,0061599 mol Sr = 0,540 g Sr.

Same for the activities of a radioactive isotope:

A(t) = A(0) * (1/2)^(t/tau)

we want the percentage of activity f the initial activities after 25 years, so that A(25)/A(0) * 100%

so the percentage is given in: (1/2)^(t/tau)

--> 1/2^(25/28,1) * 100% = 54 %