1. One atomic mass unit (amu) is 1/12 the mass of one atom of 126C.
2. 6.022 x 1023 atomic mass units (amu) = 1.000 g.
3. The masses of all other isotopes are derived by comparison to 126C, one atom of which weighs 12 amu .
4. An atomic mass (average atomic mass or atomic weight) found on the periodic chart is the weighted average mass of all the isotopes of an element. The weighing is based on the percent distribution of isotopes on earth.
5. A molecular mass (molecular weight) is the sum of all the atomic masses of all the atoms in a molecule. For example the molecular weight of water is 2(1.00794 amu) + 15.9994 amu = 18.0153 amu/molecule.
6. A formula mass (formula weight) is the sum of all the atomic weight of all the atoms in a formula. The formula weight and molecular weight of water are one in the same thing. Since ionic substances such as salts do not consist of molecules, it is best to refer to their formula weights not molecular weights. The formula weight of CaCl2 is (40.08 amu) + 2(35.45 amu) = 110.98 amu/formula unit.
7. One mol = 6.022 x 1023 just as 1 dozen = 12.
8. A molar mass of an element is the mass of one mole of average atoms of a given element in grams. It is the same number as the atomic weight but different units.
40.08 amu Ca x
1,000 g
x 6.022 x 1023 atoms = 40.08 g Ca
atom
6.022 x 1023 amu
mol
mol
9. By similar reasoning the molar mass of H2O is 18.0153 g H2O/mole and the molar mass of CaCl2 is 110.98 g CaCl2/mole.
18.0153 amu H2O
X 1.00000 g
X 6.02214 x 1023 molecules
= 18.0153gH2O
1 molecule H2O
6.02214 x 1023amu
1.00000 mole
1 mol H2O
10. An isotopic mass is the mass of one atom of a specific
isotope in atomic mass units. For example, the isotopic mass of 11H
= 1.0078 amu and that of 21H = 2.0150 amu.
Note: The atomic mass is the weighted average of naturally occurring isotopic
masses. The weighting is done according to the (%) natural abundance
on earth..