From the activity values and the decay constant, the mass of of Strontium in the sample is:

<h3>What is the decay constant of an element?</h3>
The decay constant of an element is the probability of decay of a nucleus per unit time.
{λ = ln 2 / t1/2
where;
t1/2 is the half-life of the isotope.
The half-life is converted to seconds since the decay constant is asked in per seconds.

Hence;

The activity of the element, A, the decay constant, λ and the number of nuclei, N are related as follows:
Molar mass of Strontium-90 is 90 g.
1 mole of Strontium-90 contains 6.02×10^23 nuclei.
The mass, m of Strontium in the sample is calculated:

Therefore, the mass of of Strontium in the sample is:

Learn more about decay constant at: brainly.com/question/17159453