Atomic Mass

1. Atomic Mass • Although it is possible to measure the actual mass of an atom, it is more convenient to use relative masses. • The relative mass of an atom is a comparison of the atom’s mass to the mass of a carbon-12 atom. Carbon-12 was chosen as the standard for measuring relative atomic masses.

2. Atomic Mass Unit(amu) • The mass of a carbon-12 atom = exactly 12 u • One atomic mass unit(amu or u) =1/12 the mass of a carbon-12 atom.

3. Atomic masses for each element are listed on the periodic table.

4. Some atomic masses: • Hydrogen 1.008 amu • Oxygen 16.00 amu • Iron 55.85 amu What is the atomic mass of: Nitrogen 14.01 amu Chlorine 35.45 amu Magnesium 24.31 amu

5. Formula Mass • The sum of the atomic masses of all the atoms in a chemical formula. • Formula mass can refer to an element or an ionic or molecular compound. • EX: Formula mass of CO2 is: • 1 X 12.01 + 2 x 16.00 = 44.01 u

6. What is the formula mass of Mg(NO2)2? • 1 Mg + 2 N + 4 O • 24.31 u + (2 x 14.01 u) + (4 x 16.00 u) = 116.3 u • Calculate the formula mass of: • 1) NaCl 2) NO2 3) CaCO3 • 4) Ba(NO3)2 5) (NH4)3PO4

7. Molecular Mass • molecular mass is the formula mass of a molecular substance. It is the mass of a molecule of that substance. • EX: the molecular mass of C6H12O6 = 6 X 12.01 + 12 X 1.008 + 6 X 16.00 • = 180.2 u

8. The Mole

9. PerkinElmer 2400 Series II CHNS/O Analyzer

10. Mass Spectrometer – determines molecular mass