Chemistry
Ms. Piela
A. Percent Composition: the
percentage by mass of each element
in a compound
 1. Looking at the formula for FeO, we might
assume the percent compositions of both
elements to be 50%. However, because the
percent is based on mass, the percent of Fe
is 77.7% and oxygen is 22.3% (Oxygen
weighs less)
B. Examples:
 1. Determine the percent composition of the elements
in the following compounds:
 A)
H2O
 B)
C6H12O6
 C)
Ba(NO3)2
C. Practice:
 A)
HF
 B)
NH3
II. Determining Empirical Formulas
from Percent Composition
 A. Empirical Formulas – a chemical formula that
shows the composition of a compound in terms of the
simplest ratio.
 B. Steps
 1. Convert mass percentage of each element to grams
 2. Convert grams to moles using molar mass
 3. Compare amounts of moles to find the simplest
whole-number ratios
Rhyme Time
Percent to mass
Mass to moles
Divide by smallest
Multiply until whole
C. Examples:
 Calculate the empirical formula of each of the
following substances:
 57.14 % C, 6.16% H, 9.52% N, 27.18% O
C. Examples
 48.0% C, 8.0% H, 28.0% N, 16.0% O
D. Practice
 Calculate the empirical formula of each of the
following substances:
 1. A compound is 38.77% Cl and 61.23% O.
D. Practice
 2. Magnetic iron oxide is 72.4% iron and 27.6% oxygen.
D. Practice
 3. A liquid compound is 18.0% C, 2.26% H, and 79.7%
Cl.