Determining formulae
The percentage composition of a compound
leads directly to its empirical formula.
Recall: An empirical formula for a compound
is the formula of a substance written with the
smallest integer subscripts.
Eg. Consider hydrogen peroxide:
Molecular formula = H2O2
Empirical formula = HO
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Types of Formulas
The formulas for compounds can be
expressed as an empirical formula and as a
molecular(true) formula.
Empirical
Molecular (true)
Name
CH
C 2H 2
acetylene
CH
C 6H 6
benzene
CO2
CO2
CH2O
C5H10O5
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carbon dioxide
ribose
2
Compounds with different molecular formulae
can have the same empirical formula, and
such substances will have the same
percentage composition.
Eg.
acetylene = C2H2
benzene = C6H6
both have the empirical formula = ?
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Empirical Formulas
Write your own one-sentence definition for
each of the following:
Empirical formula
Molecular formula
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• An empirical formula represents the
simplest whole number ratio of the
atoms in a compound.
• The molecular formula is the true or
actual ratio of the atoms in a
compound.
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Learning Check EF-1
A. What is the empirical formula for C4H8?
1) C2H4
2) CH2
3) CH
B. What is the empirical formula for C8H14?
1) C4H7
2) C6H12
3) C8H14
C. What is a molecular formula for CH2O?
1) CH2O
2) C2H4O2
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3) C3H6O3
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Solution EF-1
A. What is the empirical formula for C4H8?
2) CH2
B. What is the empirical formula for C8H14?
1) C4H7
C. What is a molecular formula for CH2O?
1) CH2O
2) C2H4O2
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3) C3H6O3
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Learning Check EF-2
If the molecular formula has 4 atoms of
N, what is the molecular formula if SN is
the empirical formula? Explain.
1) SN
2) SN4
3) S4N4
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Solution EF-2
If the molecular formula has 4 atoms of
N, what is the molecular formula if SN is
the empirical formula? Explain.
3) S4N4
If the actual formula has 4 atoms of N,
and S is related 1:1, then there must also
be 4 atoms of S.
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Empirical and Molecular Formulas
molar mass
=
simplest mass
a whole number = n
n = 1 molar mass = empirical mass
molecular formula = empirical formula
n = 2 molar mass = 2 x empirical mass
molecular formula =
2 x empirical formula
molecular formula
= or > empirical formula10
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Empirical
Formula
Empirical
Mass
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Molecular
Formula
Molecular
Mass
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Empirical formula from Composition
Consider the following flow-diagram:
Percent composition
Mass Composition
Number of moles of
each element
Divide by smallest number of
moles to find the molar ratios
Multiply by appropriate number to
get whole number
subscripts
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Learning Check EF-3
A compound has a formula mass of 176.0
and an empirical formula of C3H4O3. What
is the molecular formula?
1) C3H4O3
2) C6H8O6
3) C9H12O9
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Solution EF-3
A compound has a formula mass of 176.0
and an empirical formula of C3H4O3. What is
the molecular formula?
2) C6H8O6
C3H4O3 = 88.0 g/EF
176.0 g
=
2.00
88.0
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Learning Check EF-4
If there are 192.0 g of O in the
molecular formula, what is the true
formula if the EF is C7H6O4?
1) C7H6O4
2) C14H12O8
3) C21H18O12
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Solution EF-4
If there are 192.0 g of O in the
molecular formula, what is the true
formula if the EF is C7H6O4?
3) C21H18O12
192 g O
= 3 x O4 or 3 x C7H6O4
64.0 g O in EF
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Finding the Molecular Formula
A compound is Cl 71.65%, C 24.27%,
and H 4.07%. What are the empirical and
molecular formulas? The molar mass is
known to be 99.0 g/mol.
1. State mass percents as grams in a
100.00-g sample of the compound.
Cl 71.65 g
C 24.27 g
H 4.07 g
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2. Calculate the number of moles of each
element.
71.65 g Cl x 1 mol Cl = 2.02 mol Cl
35.5 g Cl
24.27 g C x
1 mol C
12.0 g C
= 2.02 mol C
4.07 g H x
1 mol H
1.01 g H
=
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4.04 mol H
18
Why moles?
Why do you need the number of moles
of each element in the compound?
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3. Find the smallest whole number ratio by
dividing each mole value by the smallest
mole values:
Cl: 2.02
=
1 Cl
2.02
C:
H:
2.02
2.02
=
1C
4.04
=
2H
2.02
4. Write the simplest or empirical formula
CH2Cl
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5. EM (empirical mass)
= 1(C) + 2(H) + 1(Cl) = 49.5
6. n = molar mass/empirical mass
Molar mass
EM
=
99.0 g/mol
49.5 g/EM
= n=2
7.Molecular formula
(CH2Cl)2
= C2H4Cl2
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Learning Check EF-5
Aspirin is 60.0% C, 4.5 % H and 35.5 O.
Calculate its simplest formula. In 100
g of aspirin, there are 60.0 g C, 4.5 g
H, and 35.5 g O.
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Solution EF-5
60.0 g C x
4.5 g H
___________= ______ mol C
x ___________ = _______mol H
35.5 g O x ___________ = _______mol O
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Solution EF-5
60.0 g C x
1 mol C
=
5.00 mol C
=
4.5 mol H
=
2.22 mol O
12.0 g C
4.5 g H
x
1 mol H
1.01 g H
35.5 g O x
1mol O
16.0 g O
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Divide by the smallest # of moles.
5.00 mol C =
________________
______ mol O
4.5 mol H
=
______ mol O
________________
2.22 mol O =
________________
______ mol O
Are are the results whole numbers?_____
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Divide by the smallest # of moles.
5.00 mol C =
___2.25__
2.22 mol O
4.5 mol H
2.22 mol O
=
___2.00__
2.22 mol O =
___1.00__
2.22 mol O
Are are the results whole numbers?_____
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Finding Subscripts
A fraction between 0.1 and 0.9 must not
be rounded. Multiply all results by an
integer to give whole numbers for
subscripts.
(1/2)
(1/3)
(1/4)
(3/4)
0.5
0.333
0.25
0.75
x2
x3
x4
x4
=
=
=
=
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1
1
3
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Multiply everything x 4
C: 2.25 mol C
x 4 = 9 mol C
H: 2.0 mol H
x 4 = 8 mol H
O: 1.00 mol O
x 4 = 4 mol O
Use the whole numbers of mols as the
subscripts in the simplest formula
C9H8O4
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Learning Check EF-6
A compound is 27.4% S, 12.0% N and
60.6 % Cl. If the compound has a molar
mass of 351 g/mol, what is the molecular
formula?
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Solution EF 6
0.853 mol S /0.853 = 1 S
0.857 mol N /0.853 = 1 N
1.71 mol Cl /0.853
= 2 Cl
Empirical formula = SNCl2 = 117.1 g/EF
Mol. Mass/ Empirical mass
351/117.1 = 3
Molecular formula = S3N3Cl6
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STRUCTURAL FORMULA
The atoms in a molecule are connected or
chemically bonded in a precise way. A SF.
Shows how the atoms in a molecule are
arranged.
For ex: H2O H-O-H
C2H6
CH3
H-C- C- H
H H
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Empirical formula
•The simplest whole number ratio of atoms of
elements in a compound, described with the use of
subscripts.
•Ionic compounds are always shown as empirical
formulas.
Molecular Formula
The actual numbers of atoms in a molecule.
Structural Formula
Show the relative arrangements of atoms in a
molecule
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HYDRATES
Solids which are found in combined
form with water in definite proportion
are called as HYDRATES. When
hydrates are heated, H2O evaporates,
and only solid is obtained in
amorphous .
(w/o a certain geometric structure,
generally in powdered form. H2O
molecules surround ionic substances
with certain amounts.
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WATER OF HYDRATION : Water molecules of a hydrate.
• Na2CO3.10H2O
Na2CO3(s) + 10H2O(g)
• DEHYDRATION: Evaporation of water
• Na2CO3.10H2O,
of hydration.
• CaSO4.2H2O,
• CuSO4.5H2O
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