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EMPIRICAL AND MOLECULAR FORMULAS
An SCH 2D1 Production
NO
BrO
O
C18H20BrNO3
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Today’s Agenda Calculating Percent Composition Determining Chemical Formulas
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Remind Me... What was the law of conservation of
mass?
What was the law of definite proportions?
What is percent composition?
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Calculating the Percentage Composition
METHOD 1: Chemical Formula is Unknown A 24.5 g sample of an unknown hydrocarbon (CxHy) is
decomposed to yield 20.2g of pure carbon and some hydrogen gas. Calculate the percent composition of this hydrocarbon.
A 24.5 g sample of an unknown hydrocarbon (CxHy) is decomposed to yield 20.2g of pure carbon and some hydrogen gas. Calculate the percent composition of this hydrocarbon.
CxHy C(s) + H2 (g)
24.5g 20.2g ?
Percent Composition?
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Step 1: Mass % of Carbon = mass of C x 100 mass of compound = 20.2 x 100 24.5= 82.4%
Calculating the Percentage Composition
CxHy C(s) + H2 (g)
24.5g 20.2g ?mass reactants = mass productsmhydrocarbon = mcarbon + mhydrogen mhydrogen = 24.3 – 20.2
= 4.3g
Therefore, the % composition is 82.4% C and 17.6% H
Mass % of Hydrogen = mass of H x 100 mass of compound = 4.3 x 100 24.5= 17.6%
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Calculating the Percentage Composition
METHOD 2: Chemical Formula Known What is the % composition of the amino
acid alanine (C3H7NO2)?
% element = total mass of each element molar mass of the compound
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Calculating the Percentage Composition
% element = total mass of each element
molar mass of the compound% Carbon =
(3x 12.0)
89.0X 100 = 40.4 % C
% Hydrogen =(7x 1.01)
89.0X 100 = 7.9 % H
% Nitrogen =(1x 14.0)
89.0X 100 = 15.7 % N
% Oxygen =(2x 16.0)
89.0X 100 = 36.0 % C
C3H7NO2
100%
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Determining Chemical Formulas
Law of Multiple Proportions (LMP) Elements combine in simple whole
number ratios to form compounds
Empirical (Simple) Formula Simplest whole number ratio of the
elements in a compound.
Molecular Formula Actual number of atoms in one
molecule or formula unit of the compound
1:1
2:1
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Determining Chemical Formulas
C6H6 CHMolecular formula
Empirical formula
Benzene
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Determining Chemical Formulas
Compound
Molecular Formula
Lewis Structure
Empirical Formula
Ratio ofElements
Water
Hydrogenperoxide
Oxalic acid
Propene
Cyclopropane
HO
HH
OOHO
C CO
O O
H H
C C CH
H
HH
H H
CC C
H H
HH
HH
H2O
HO
CHO2
CH2
CH2
2:1
1:1
1:1:2
1:2
1:2
H2O
H2O2
C2H2O4
C3H6
C3H6
Substances with the same molecular formula but different structures are called
structural isomers!
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Given that a compound contains 12.8% C, 2.1% H and 85.1% Br, calculate the empirical formula.
Calculate the molecular formula of the compound if its molar mass is 190 g/mol.
Empirical & Molecular Formula