1. Atoms and Molecules Formula Unit Mass The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.

2. Atoms and Molecules Formula Unit Mass The method in which formula unit mass and molecular mass is calculated remains the same. The only difference remains that molecular mass is calculated for a molecule of any element, while formula unit mass is calculated for those compounds whose constituent elements are ions.

3. Atoms and Molecules Formula Unit Mass Molecular mass is calculated for H 2 while formula unit mass is calculated for CaCl 2. Molecular mass is calculated for H 2 while formula unit mass is calculated for CaCl 2.

4. Atoms and Molecules Formula Unit Mass How to Find Formula Unit Mass  Write a correct formula for the compound.  Look up the atomic mass of each element from the periodic table in the compound and round it to two decimal places.  Multiple the rounded atomic mass by the subscripts, if any.  Add all masses of elements together and use the unit, g/mol.

5. Atoms and Molecules Formula Unit Mass Example Find the formula mass of sodium chloride. Formula: NaCl Na = 23.00 (mass) X 1 (how many) = 23.00 Cl = 35.45 (mass) X 1 (how many) = 35.45 Total = 58.45 g/mol Thus, mass of one mole of sodium chloride = 58.45 grams

6. Atoms and Molecules Formula Unit Mass Example Find the formula mass of calcium phosphate. Formula: Ca 3 (PO 4 ) 2 Ca = 40.08 (mass) X 3 (how many) = 120.24 P = 30.97 (mass) X 2 (how many) = 61.94 O = 16.00 (mass) X 8 (how many) = 128.00 Total = 310.18 g/mol Thus, mass of one mole of calcium phosphate = 310.18 grams

7. Atoms and Molecules Formula Unit Mass Example Calculate the formula mass of calcium chloride. Formula: CaCl 2 Ca = 40.08 (mass) X 1 (how many) = 40.08 Cl = 35.45 (mass) X 2 (how many) = 70.90 Total = 110.98 g/mol Thus, mass of one mole of calcium chloride = 110.98 grams