Element X has two naturally occurring isotopes, 35X and 36X . 35X has a percent natural abundance of 30%, while 36X has a percent natural abundance of 70%. What is the average atomic mass of element X

= 35.7 amu

Explanation:

            Isotopic                fractional                                Wt Avg mass

          mass (amu)      abundance (= % / 100)       = Isotopic mass X (= % / 100)

X-35          35                         0.30                                           10.5 amu

X-36          36                         0.70                                           25.2 amu

Average Atomic Mass of Element X = ∑ Wt. Avg. Masses = 35.7 amu

14 days is the half-life of phosphorus-32