Radioactive decay is first order in the decaying isotope. For example, strontium-90 contained in fallout from nuclear explosions decays to yttrium-90 and a beta particle. Write the rate law for the decay of strontium-90.

Can you please take me step by stem through this problem?

3 answers

rate = k(Sr-90)
What is the rate
also what is k and Sr i am so confused
For a first order reaction
rate = k*(concentration)^1
k is a constant.
Your question asks for the expression, not for an answer numerically. They don't give a k and they don't list a concentration.
k can be determined from
k = 0.693/t1/2 and you can evaluate that by looking up the half life of Sr-90 and plugging it into the equation.