Mass Relationships in
Chemical Reactions
Chapter 3
Formula Weight (FW)
• Sum of the atomic weights for the atoms in a
chemical formula.
• e.g., formula weight of calcium chloride, CaCl2
Ca: 1(40.1 amu)
+ 2Cl: 2(35.5 amu)
111.1 amu
• Formula weights used for ionic or molecular
compounds
Molecular Weight (MW)
• Sum of the atomic weights of the atoms in a
molecule.
• For the molecule ethane, C2H6, the molecular weight
would be
C:
+ 6H:
2(12.0 amu)
6(1.0 amu)
30.0 amu
• Formula weights used for molecular compounds only
Percent Composition
Percentage mass of a compound that
comes from each of the elements in the
compound:
(number of atoms)(atomic weight)
% element =
(FW of the compound)
x 100%
Percent Composition
For the molecule ethane, C2H6, with MW = 30.0 amu
(2)(12.0 amu)
%C =
=
(30.0 amu)
24.0 amu
30.0 amu
x 100%
= 80.0% C in ethane
Micro World
(atoms & molecules)
Macro World
(grams)
The mole (mol) - amount of a substance that
contains as many particles as there
are atoms in exactly 12.00 grams of 12C
1 mol = 6.02 x 1023
Avogadro’s number (NA)
Avogadro’s Number
• 1 carbon atom has a mass of 12.0 amu
• 1 mole of 12C has a mass of 12.0 g
Micro World
(atoms & molecules)
Macro World
(grams)
Using Moles
• Moles provide a bridge from the molecular scale
to the real-world scale.
How many moles of calcium (Ca) are in
5.0750 g of calcium? (Z = 20)
From periodic table: 1 mol Ca = 40.078 g Ca
 1 mol Ca 
  0.12263 mol Ca
5.0750 g Ca
 40.078 g Ca 
atoms
ions
Molar mass - the mass of 1 mole of molecules in grams
formula units
1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g
1 mole S atoms = 32.066 g S
1 mole lithium atoms = 6.941 g of Li
For any element:
atomic mass (amu) = molar mass (grams)
(Average) atomic mass (6.941)
Do You Understand Molar Mass?
How many moles are in 0.551 g of potassium (K) ?
1 mol K = 39.0983 g K
1 mol K
0.551 g K x
= 1.41 x 10-2 mol K
39.0983 g K
Molecular mass (or molecular weight) - the sum of
the atomic masses (in amu) in a molecule.
1S
SO2
2O
SO2
32.07 amu
+ 2 (16.00 amu)
64.07 amu
For any molecule:
molecular mass (amu) = molar mass (grams)
1 molecule SO2 = 64.07 amu
1 mole SO2 = 64.07 g SO2
Do You Understand Molar Mass?
How many moles are in 72.5 g of C3H8O ?
1 mol C3H8O = (3 mol C) (12.0 g/mol)
+ (8 mol H) (1.0 g/mol)
+ (1 mol O) (16.0 g/mol)
1 mol C3H8O = 60.0 g C3H8O
1 mol C3H8O
(72.5 g C3H8O)
60.0 g C3H8O
= 1.21 mol C3H8O
Do You Understand Molar Mass?
What is the mass of 0.433 mol of Ca(NO3)2 ?
1 mol Ca(NO3)2
=
(1 mol Ca) (40.078 g/mol)
+ (2 mol N) (14.01 g/mol)
+ (6 mol O) (16.00 g/mol)
=
0.433 mol Ca(NO3)2
164.1 g Ca(NO3)2
164.1 g Ca(NO3)2
·
1 mol Ca(NO3)2
= 71.1 g Ca(NO3)2