AP Chemistry A. Allan Chapter 3 Notes - Stoichiometry 3.1 Atomic Masses A. C-12, the Relative Standard 1. C-12 is assigned a mass of exactly 12 atomic mass units (amu) 2. Masses of all elements are determined in comparison to the carbon - 12 atom (12C) the most common isotope of carbon 3. Comparisons are made using a mass spectrometer B. Atomic Mass (Average atomic mass, atomic weight) 1. Atomic masses are the average of the naturally occurring isotopes of an element 2. Atomic mass does not represent the mass of any actual atom 3. Atomic mass can be used to "weigh out" large numbers of atoms 3.2 The Mole A. Avogadro's number 1. 6.022 x 1023 units = 1 mole 2. Named in honor of Avogadro (he did NOT discover it) B. Measuring moles