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 1:1Mass 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 hydrogen1) Divide the mass of each element by the total mass of the sample

and multiply by 100%

2) Subtract known percentages from 100 to determine percent composition of oxygen

3) 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

2) Divide mass of each element by total mass and multiply by 100%

3) Check that percentages add up to 100%

4) Include a statement to summarize results

Practice1) 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