Percent Composition
Objective-calculate the percent composition of compounds
Law of Definite Proportion- Compounds are consistent chemical combinations of
atoms that can be expressed as

Ratio of masses

Ratio of atoms

Ratio of moles of atoms

Percent composition
% mass 
grams of element in 1 mole compound
x100
molar mass of compound
Example: Periodic Table
Calculate the percent Nitrogen in Dinitrogen Monoxide N2O
PRACTICE
Calculate the percent composition of each element in magnesium hydroxide.
Percent Mg
Percent O
Percent H
24.3
32.0
2.0
x100  42%
x100  55%
x100  3.4%
58.3
58.3
58.3
How is math utilized to describe chemical formulas?
T. O’Toole
Example: Lab Data
9.03 grams of Mg combine completely with 3.48 grams of N to form a compound. What
is the percent Mg in this compound?
Example: Application
Calculate how many grams of hydrogen are in 20.2 grams of Sodium Bisulfate
NaHSO4
.0083(20.2)  .17 g
How is math utilized to describe chemical formulas?
T. O’Toole
Minute Paper
Potassium Chlorate decomposes to form oxygen and potassium chloride. Use percent
composition to determine the mass of oxygen formed from 15.0 grams of potassium
chlorate.
How do I get from where I am to where I’m going?
The 10 Minute Makeover
Customize your learning by choosing from the following options each night
 Complete the appropriate section on the Portfolio Page
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 Read pages 188-192 in the book
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 Check out the Extra Practice and the Tutorial Resources in the LiveBinder
 Review the skills from today’s Train Your Brain
How is math utilized to describe chemical formulas?
T. O’Toole