Percent Composition and Empirical Formula

Dr. Davis
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)
 B)
 C)
C. Practice:
 A)
 B)
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
C. Examples:
 1. Chemical analysis of a liquid shows that it is 60.0%
C, 13.4% H, and 26.6% O by mass. Calculate the
empirical formula of this substance.
C. Examples
 2. A dead alkaline battery is found to contain a
compound of Mn and O. Its analysis gives 69.6% Mn
and 30.4% O. Calculate the empirical formula of this
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%