Percent Composition
Objectives:
• Calculate the percent
composition of compounds
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Law of Definite Proportions
• 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
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Percent Composition
grams of element in 1 mole compound
% mass =
x100
molar mass of compound
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EXAMPLE: Periodic Table
• Calculate the percent Nitrogen in
Dinitrogen Monoxide
N 2O
2(14.0) = 28.0
grams of nitrogen in 1 mole
molar mass of compound
2(14.0) + 16.0 = 44.0
Percent Composition
28.0
x100 = 63.6%
44.0
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Percent Composition
What is the % composition of each element in
magnesium hydroxide?
Mg(OH)2 =24.3 + 2(16.0 + 1.0)= 58.3 g
24
2.03
32
100==41
3.4.79%
xx100
%
54
58..33
58
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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?
mass of compound
Percent Composition of Mg
9.03 + 3.48 = 12.51
9.03
x100 = 72.2%
12.51
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EXAMPLE: Application
• Calculate how many grams of hydrogen are
in 20.2 grams of Sodium Bisulfate
grams of hydrogen in 1 mole
NaHSO4
.0083(20.2) = .17 g
1(1.0) = 1.0
molar mass of compound
1(23.0) + 1(1.0) + 1(32.1) + 4(16.0) = 120.1
Percent Composition
1.0
= .0083%
120.1
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