Final answer:
To find the mass of 90Sr deposited on the surface of the earth, convert the specific activity from MCi to Bq and use the formula for radioactive decay to calculate the initial amount. Then, multiply the initial amount by the molar mass of 90Sr to find the mass.
Step-by-step explanation:
To find the mass of 90Sr, we need to use the concept of radioactivity and decay. The activity of a radioactive substance is measured in Becquerel (Bq), which corresponds to the number of decays per second. The specific activity of 90Sr is given in millicuries (MCi), which represents the number of decays per second in one millicurie. To convert MCi to Bq, we can use the conversion factor 1 Ci = 3.7 x 10^10 Bq. So, 9 MCi is equal to 9 x 3.7 x 10^10 Bq.
The half-life of 90Sr is 28.6 years, which means that half of the 90Sr will decay in that time. Using the formula for radioactive decay, N(t) = N0 * (1/2)^(t/T), where N(t) is the final amount, N0 is the initial amount, t is the time, and T is the half-life, we can calculate the initial amount of 90Sr.
Once we have the initial amount, we can use the atomic mass of strontium (which is 87.62 g/mol) to find the mass of 90Sr. The molar mass of 90Sr is 89.91 g/mol, since it has one more neutron than the stable isotope. Therefore, the mass of 9 MCi of 90Sr is equal to the initial amount multiplied by the molar mass, which gives us the answer.