Percent Composition
LG: I can calculate percent composition given
a chemical formula or experimental data
Percent Composition
• When unknown substances undergo chemical
analysis, the results are reported as percent
composition, which is the percentage of the
total mass made up by each element
– The fertilizer below contains 18% nitrogen, 24%
phosphorous, and 6% potassium
• Percent composition can be
determined by experimental data
or from a chemical formula
Law of Definite Proportions
• Percent composition is useful in identifying unknown
substances because all chemical compounds abide
by the law of definite proportions: “The elements in
a given compound are always present in the same
proportion by mass”
– Example:
Water
Hydrogen
Peroxide
Formula
H2O
H2O2
Ratio of atoms
2:1
1:1
Mass ratio
2:16 or 1:8
2: 32 or 1:16
• Example 1: Use the following data to determine percent
composition
– A 500.00 mg Aspirin tablet, C9H8O4, contains 300.00 mg of carbon and
8.08 mg of hydrogen
1) Divide the mass of each element by the total mass of the
sample and multiply by 100%
1) Subtract known percentages from 100 to determine percent
composition of oxygen
1) Include a statement to summarize results
• Example 2: Calculate the percent composition of each
element in Mg(OH)2
1) Find molar mass of compound
1) Divide mass of each element by total mass and multiply by
100%
1) Check that percentages add up to 100%
1) Include a statement to summarize results
Practice
1) Determine the percent
composition for potassium
sulfate
2) Determine the percent
composition of a 2.50 g sample
is found to contain 0.900 g of
calcium and 1.60 g of chlorine
Mixtures and Alloys
• Percent composition can also be used to
analyze:
– Alloys: different metals melted and blended
together
– Mixtures: Food chemists are responsible for
verifying nutrition information on labels (ie. Salt
content)
Homework
• Old Book – Pg. 179 # 1-3 Pg. 184 # 5-8
• New Book – Pg. 288 # 1, 2, 5, 6, 7
• Review lab procedure for tomorrow and
complete pre-lab calculation