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Atomic mass unit (AMU) The masses of protons and neutrons are nearly equal (1.0073 and 1.0087 amu respectively). Avogadro’s Number and the mole A mole is the amount of matter that contains as many objects (atoms, molecules, or other objects) as the number of atoms in 12 grams of 12 C . This number has been determined through experimentation to be 6.022 ! 10 23 , and is known as Avogadro’s number. A mole of atoms, a mole of molecules, or a mole of anything all contain Avogadro’s number of these objects: 1 mol 12C atoms = 6.022 ! 10 23 12C atoms 1 mol H 2O molecules = 6.022 ! 10 23 H 2O molecules Ex: Convert 1.8 ! 10 –11 J J to , where U = Uranium. U nucleus mole of U % J J " %" –11 23 U nuclei 13 $# 1.8 ! 10 1 U nucleus '& $ 6.022 ! 10 mole of U ' = 1.08 ! 10 mole of U # & The mass of a single atom of an element [amu] is numerically equal to the mass [g] of 1 mol of that element, regardless of the element. 1 atom of 12 C has a mass of 12 amu ! 1 mol 12 C has a mass of 12 g. Molar mass The molar mass of an element is the atomic weight in grams per mole, i.e. the molar mass [g/mol] of any substance is always numerically equal to its formula weight [amu] e.g. 1 H2O molecule has a mass of 18.0 amu ↔ 1 mol H2O has a mass of 18.0 g 1 O2 molecule has a mass of 16.0 amu ↔ 1 mol O2 has a mass of 16.0 g Q: How many moles of water are in a liter of water, if 1 liter = 1 kilogram water. A: Atomic weights: Hydrogen: 1.008 amu Oxygen: 16.00 amu Formula weight = 1.008 amu + 1.008 amu + 16.00 amu = 18.016 amu ! 1 mol $ 1000 g # = 55.55 moles of water " 18.0 g &% References Chemistry, the Central Science. Brown et al., Pearson