Atomic Masses. (6 pts) An element has two naturally occurring isotopes: Isotope A has a mass of 120.9038 amu and a relative abundance of 57.4%; isotope B has a mass of 122.9042 amu. What is this element, and what is its atomic mass? 34. Grams and Moles. (4 pts) What is the mass of 15 moles of CO2 in grams?