please help question #2

2. One Step Further: Suppose 1.0109mol of radioiodine were released into a water reservoir after a nuclear accident. Approximately how many atoms of radioactive 131 I would remain in the reservoir one year ( 365 days) after the release? 131 is a radioactive isotope (or radioisotope) of iodine with 53 protons and 78 neutrons in its nucleus. Its nuclear decay reaction proceeds by a first-order process that produces a 131 Xe nucleus, a particle (electron), an antineutrino (ve) and high-energy gamma rays: 53131I54131Xe+e+ve+t1/2=8.02days High exposure to 131 I radioactivity can cause damage to the thyroid, which is the main organ in the body that uses iodine. This can be a health danger after a nuclear disaster like the one at Fukushima, Japan in 2011 , because 131 is produced by the fission of uranium and plutonium and can contaminate air and water sources. For thyroid cancer patients, however, this effect can be a blessing in disguise-when they take a prescribed dose of 131, the thyroid cancer cells naturally absorb more of the radioactive element, which then kills the cancer with little damage to the rest of the body