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Chapter Outline
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7.1 Chemical reactions and the Conservation of Mass
7.2 Balancing Chemical Equations
7.3 Combustion Reactions
7.4 Stoichiometric Calculations and the Carbon Cycle
7.5 Percent Composition and Empirical Formulas
7.6 Empirical and Molecular Formulas Compared
7.7 Combustion Analysis
7.8 Limiting Reactants and Percent Yield
Percent Composition: the composition of a compound in terms
of the percentage by mass of each element in the compound
n x molar mass of element
x 100%
molar mass of compound
n is the number of moles of the element in 1 mole
of the compound
3 x (1.008 g)
x 100% = 3.086%
97.99 g
1 x (30.97 g)
%P =
x 100% = 31.60%
97.99 g
4 x (16.00 g)
%O =
x 100% = 65.31%
97.99 g
%H =
H3PO4 M = 98.00
g/mol
Sample Exercise 7.4 – Calculating Percent
Composition from a Chemical Formula
The mineral forsterite: Mg2SiO4
3.086% + 31.60% + 65.31% = 99.996%
= 100.00%
Empirical Formulas
A formula showing the smallest whole number
ratio of elements in a compound, e.g.
Benzene:
» Empirical = CH
» Molecular = C6H6
Glucose
» Empirical = CH2O
» Molecular = C6H12O6
Empirical Formula from % Composition
Sample Exercise 7.6
A sample of the carbonate mineral dolomite is 21.73% Ca, 13.18%
Mg, 13.03% C, and the rest is oxygen. What is its empirical formula?
Assume 100 g and
divide by atomic
weight
Divide by
fewest number
of moles
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Empirical and Molecular
Formulas Compared
Chapter Outline








7.1 Chemical reactions and the Conservation of Mass
7.2 Balancing Chemical Equations
7.3 Combustion Reactions
7.4 Stoichiometric Calculations and the Carbon Cycle
7.5 Percent Composition and Empirical Formulas
7.6 Empirical and Molecular Formulas Compared
7.7 Combustion Analysis
7.8 Limiting Reactants and Percent Yield
The molecular formula can be determined if the
molecular weight of the compound is known.
empirical = CH2O
molecular = C2H4O2
Note that molecular = empirical x 2
glycolaldehyde
C2H4O2 = (CH2O)2
Or more generally –
molecular = (empirical)n
Empirical and Molecular
Formulas Compared
Molecular Mass and Mass Spectrometry
The molecular formula can be determined if the molecular
weight is known.
e.g. glucose
Acetylene
C2H2
Benzene
C6H6
Sample Exercise 7.7 – Using Percent Composition and
Molecular weight to Derive a Molecular Fomula
Pheromones are chemical substances secreted by members of a
species to stimulate a response in other individuals of the same
species. The percent composition of eicosene, a compound similar to
the Japanese beetle mating pheromone, is 85.63% C and 14.37% H.
Its molecular mass, as determined by mass spectrometry, is 280 amu.
What is the molecular formula of eicosene?
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