Percent Composition – Notes
 Key Vocabulary
Formula mass – the sum of the average atomic masses of all atoms
represented in a molecule’s formula
Percentage composition – the percentage by mass of each element in a
compound
 Input
Formula Masses (amu)
To find the formula mass find the amu (atomic mass unit) of each
element in the formula. Next multiply the amu for each element by the
number of each element in the formula. Finally add them all together.
Examples: H20 has the elements hydrogen (amu = 1.01) and oxygen (amu =
16.00).
(2 x 1.01) = 2.02 for hydrogen then add 16.00 for oxygen.
Finally the average mass for H20 is 18.02 amu
Molar Masses (g/mol)
Molar mass is calculated exactly the same as formula mass. The
difference is that molar mass units are grams per mole while formula mass
units are amu’s.
Conversions
Grams  use molar mass  moles
Moles  use Avogadro’s number  formula units
Examples: How many copper atoms in a penny that weighs 3g?
Grams  use molar mass  moles
3g Cu(1 mol/63.5 g Cu) = .047244 moles of Cu
Moles  use Avogadro’s number  formula units
.047244 moles of Cu (6.022 x 1023 atoms) = 2.84 x 1022 Cu atoms
How many glucose molecules (C6H12O6) are in 5.23 g of glucose?
Answer = 1.75 x 1022 molecules C6H12O6
How many oxygen atoms are in the glucose sample?
Answer = 1.05 x 1023 atoms O
Percentage Composition
To find the percentage of mass of an element in a compound
First find the molar mass of the compound
Second divide the mass of the element by the mass of the compound
Third multiply by 100 to get the percentage
Examples: Find the percentage of chlorine in lead (II) chloride (PbCl 2)
Pb = 207.2 g
Cl = 35.5 g times 2 atoms = 71 g
PbCl2 = 278.2g
71g divided by 278g = .2554
Multiply by 100 = 25.5% chlorine
Find the mass percentage of water in the hydrate CuSO4*5H2O
Answer = 36.08%
Find the percentage composition of the following Ba(NO3)2
Answer = 52.55% Ba, 10.72% N, 36.73% O
 Guided Practice:
Sample problems p 238 (Finding formula mass)
Sample problems p 239 (Finding molar mass)
Sample problems p 240 - 242 (Grams)
Sample problems p 243 - 244 (Percent composition)
 Resources
Modern Chemistry text (Holt, Rinehart, and Winston) Page 237 – 244