Molar Mass and Percent Composition
Name____________________________
Date ___________
Determine the formula mass for each of the following compounds. Individual atomic masses can be found on your
periodic chart. You must show all work here or on another sheet of paper.
1 sulfur atom in the sulfate ion,
and the 3 outside is distributive
across ( ), so there are 3 atoms of
sulfur in the compound.
Find Quantity of
atoms
Atomic mass from periodic table
Example: Find formula mass of Al2(SO4)3
2
Al
x 26.98 =
54.0 g
3
S
x 32.1 =
96.3 g
12
O
x 15.999 =
192.0 g
Property is distributive, so 3 x 4 = 12!
M.M. = 342.3 g
amu
Mass contribution for
Each element
Add the mass contribution of
each element.
Names, molar mass, and percent of component indicated:
1. K2S
(%K)
Potassium sulfide = 110.266 g/mol
K = 71%
2. Pb(ClO4)2
(% ClO4)
Lead (II) perchlorate = 406.09 g/mol
ClO4 = 48.98%
3. KH2PO4
(%O)
Potassium dihydrogen phosphate = 136.0798 g/mol
O = 47%
4. Ca3(PO4)2
(%Ca)
Calcium phosphate = 310.172 g/mol
Ca = 38.8%
5. NiCl2
(%Ni)
Nickel (II) chloride = 129.59 g/mol
Ni = 45.3%
6. Fe(NO3)3
(%NO3)
Iron (III) nitrate = 241.862 g/mol
NO3 = 76.9%
7. CoCl2●6H2O
(%H2O)
Cobalt (II) chloride hexahydrate = 237.9188 g/mol H2O = 45.4%
8. CuCl2●2H2O
(%H2O)
Copper (II) chloride dihydrate = 170.4796 g/mol
H2O = 21.2%
9. K2SO4●10H2O
(%H2O)
Potassium sulfate decahydrate = 354.41 g/mol
H2O = 50.8%
10. Cd(NO3)2●4H2O
(%H2O)
Cadmium nitrate tetrahydrate = 308.4772 g/mol
H2O = 23.4%