A(t) = amount remaining in t years = A0ekt, where A0 is the initial amount and k is a constant to be determined. Since A(1690) = (1/2)A0 and A0 = 80, we have 40 = 80e1690k 1/2 = e1690k ln(1/2) = 1690k k = -0.0004 So, A(t) = 80e-0.0004t Therefore, A(430) = 80e-0.0004(430) = 80e-0.172 ≈ 67.4 g
A. Oxygen-18 is the correct reference for this isotope. As this notation requires the name of the element followed by a dash and the mass number of that corresponding element. The mass number is obtained by adding the number of protons and neutrons together as in this example.