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III. Moles/Stoichiometry
In all chemical reactions there
is a conservation of mass,
energy, and charge.
•Matter cannot be created nor destroyed, only changed from
one form to another
•Energy cannot be created nor destroyed, only changed from
one form to another
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Types of chemical reactions
include synthesis, decomposition,
single replacement, and double
replacement.
 Synthesis – one product forms from several
reactants
 Decomposition – several products form from one
reactant
 Single replacement – an element on table J
replaces another element in a compound that is
lower on table J
 Double replacement – two compounds for two
different compounds by switching ions
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Examples of different types of reactions
 Synthesis: A + B
C
– Na(s) + Cl2(g)
2NaCl(s)
 Decomposition: C
A+B
– 2KClO3(s)
2KCl(s) + 3O2(g)
 Single replacement: AB + C AC + B
– Zn(s) + CuSO4(aq)
ZnSO4(aq) + Cu(s)
 Double replacement: AB + CD AD + BC
– AgNO3(aq) + KCl(aq)
KNO3(aq) + AgCl(s)
Check Table F for insoluble compounds that
form precipitates when solutions are mixed.
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Precipitate – not
soluble in water
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A balanced chemical equation
represents conservation of
atoms. The coefficients in a
balanced chemical equation can
be used to determine mole ratios
in the reaction.
•A mole of molecules is made up of 6.02 x 1023 molecules
•A chemical equation is balanced to ensure the conservation
of matter. The types and number of atoms on each side of the
equation (before and after the reaction) must be equal.
•The total mass before the reaction must equal the total mass
after the reaction has taken place.
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Chemical equations must be
balanced so that mass can be
conserved.
Word equation: hydrogen + oxygen
Chemical equation: 2H2 + O2
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water
2H2O
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How many on the left? How
many on the right? What do I do?
 Balance the reaction Fe + O2
Fe2O3
– There are 2 oxygen atoms on the left and 3 on the right.
To get them equal I need to multiply the left by 3 and
the right by 2. These multiples are called coefficients
and are placed in front of the formula. The yield sign
and the plus separate the formulas.
 Fe + 3O2
2Fe2O3
– Now the number of iron atoms has to be balanced.
There is one iron on the left and four on the right. Use a
coefficient of 4 in front of the Fe on the left.
 4Fe + 3O2
2Fe2O3
– Now it is correctly balanced
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A more active metal will
replace a less active
metal from its
compound during a
single replacement
reaction.
 Zn + CuSO4 Cu + ZnSO4
Zinc replaces copper because zinc is
more active than copper.
 Cu + ZnSO4
No Reaction
Copper cannot replace zinc
Hydrogen is on Table J to show which metals will react
with acids (HCl, H2SO4, HNO3) to produce hydrogen.
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Zn + 2HCl
ZnCl2 + H2
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Regents Question: 01/03 #46
According to Reference Table J, which of these metals
will react most readily with 1.0 M HCl to produce
H2(g)?
(1) Ca
(2) K
(3) Mg
(4) Zn
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A single replacement reaction
will take place when:
 A more active metal replaces a less active metal
from its compound
 A more active nonmetal replaces a less active
nonmetal from its compound
 Criss Cross to find the new formula
 Check Table J for the activity of metals and
nonmetals.
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Zn +PbCl2
ZnCl2 + Pb
Pb + ZnCl2
No Reaction
Zinc is more
active than lead
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The oxidation states of the
elements are available on the
Periodic Table of the Elements
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Check Table F to see if there is a precipitate
during a double replacement reaction.
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AgCl is a precipitate because it is insoluble in water.
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Polyatomic ions are found on
Reference Table E
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To find the formula of a compound,
use the criss-cross method
 Write the symbol and oxidation state of the metal
(lower electronegativiy if both are nonmetals) in
the upper right hand corner of the symbol.
Positive oxidation state is written first.
 Repeat for nonmetal (-)
C4+ O2 Drop the sign
C4
O2
 Criss Cross
C2
O4
 Reduce if possible
CO2
Although Carbon and Oxygen are both nonmetals, Carbon has a lower
electronegativity and therefore uses a positive oxidation number
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A double replacement reaction
will take place when:
 The reactants are in solution and one of the
products is not soluble in water
 Check Table F for solubility rules
Notice: the metal
comes first
–A B+C D
AD + CB
– A combines with D and B combines with C
– Criss Cross to find the new formulas
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When using the criss-cross method with
polyatomic ions, use parenthesis ( )
 Write the symbol and oxidation state of the metal (+ ion)





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in the upper right hand corner of the symbol.
Repeat for nonmetal (- ion)
(NH4) 1+ (PO4) 3Drop the sign
(NH4) 1 (PO4) 3
Criss Cross
(NH4) 3 (PO4) 1
Reduce if possible (numbers outside parenthesis only)
If there is no number following the parenthesis, drop the
parenthesis.
(NH4)3 PO4
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A compound is a substance
composed of two or more different
elements that are chemically
combined in a fixed proportion. A
chemical compound can be broken
down by chemical means (not
physical means.) A chemical
compound can be represented by a
specific chemical formula and
assigned a name based on the
IUPAC system.
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Compounds are electrically neutral.
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A chemical formula is both qualitative
and quantitative. It tells which
elements are in the compound with
symbols and how many of each with
subscripts
The formula for sulfuric acid is H2SO4
Element
Number of atoms
H
2
S
1
O
4
The subscript of 1 is never written
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Compounds have more than one
capital letter in the formula
Binary Compounds – Composed of just two elements
(2 capital letters)
•Water
H2O
2 elements
3 atoms
•Methane
CH4
2 elements
5 atoms
•Ammonia
NH3
2 elements
4 atoms
•Aluminum oxide
Al2O3
2 elements
5 atoms
Other Compounds -more than 2 elements
(more than 2 capital letters)
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•Potassium hydroxide
KOH
3 elements
3 atoms
•Calcium sulfate
CaSO4
3 elements
6 atoms
•Ammonium phosphate
(NH4)3PO4 4 elements
20 atoms
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Regents Question: 08/02 #6
Which species represents a chemical compound?
(1) N2
(2) Na
(3) NH4+
A compound is made up of 2 or
more different elements.
A compound is electrically neutral.
(4) NaHCO3
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Regents Question: 08/02 #9
If an equation is balanced properly, both sides of the
equation must have the same number of
(1) atoms
(2) coefficients
2H2 + O2
2H2O
(3) molecules
(4) moles of molecules
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Regents Question: 06/03 #59
Given the reaction between two different elements in the
gaseous state
Box A below represents a mixture of the two reactants
before the reaction occurs.
The product of this reaction is a gas. Draw the system
after the reaction has gone to completion, based on
the Law of Conservation of Matter.
One reactant
is in excess.
Box A – System before
reaction
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Box B – System after
reaction
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Regents Question: 08/03 #47
Given the unbalanced equation:
_Al + _CuSO4
_Al2(SO4)3 + _Cu
When the equation is balanced using the smallest
whole-number coefficients, what is the
coefficient of Al?
(1) 1
(2) 2
(3) 3
(4) 4
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Regents Question: 01/03 #48
Given the unbalanced equation:
_Fe2O3 + _CO
_Fe + _CO2
When the equation is correctly balanced using the
smallest whole-number coefficients, what is the
coefficient of CO?
(1) 1
(2) 2
Try a 3 in front of CO2
(3) 3
(4) 4
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Regents Question: 06/03 #9
Which substance can be decomposed by a chemical
change?
(1) Co
(2) CO
(3) Cr
(4) Cu
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The formula mass of a
substance is the sum of the
atomic masses of its atoms.
The molar mass (gram
formula mass) of a substance
equals one mole of that
substance.
Formula mass of H SO
H 2x1 = 2
2
4
S 1 x 32 = 32
O 4 x 16 = 64
98 amu
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The formula mass represents the
mass of one molecule of a
substance while the gram
formula mass represents the
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mass of a mole (6.02 x 10
molecules) of that substance.
 The calculation for formula mass and gram
formula mass (GFM) are the same, the difference
is in the units.
If they tell you to use the mass
– Formula Mass of H2SO4 = 98 amu
– Gram Formula Mass of H2SO4 = 98 g
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rounded to the nearest tenth
then use tenths for each mass as
well as your final answer.
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Regents Question: 06/02 #41
The gram formula mass of NH4Cl is
(1) 22.4 g/mole
(2) 28.0 g/mole
N 1 x 14.0 =
(3) 53.5 g/mole
H 4 x 1.0 =
(4) 95.5 g/mole
Cl 1 x 35.5 = _______
g/mole
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Regents Question: 06/02 #34
A compound has a gram formula mass of 56 grams per
mole. What is the molecular formula for this
compound?
(1) CH2
(2) C2H4
(3) C3H6
(4) C4H8
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Grams/GFM = Moles
To convert grams into moles, divide the grams by
the gram formula mass
To convert moles into grams, multiply by the moles
by the gram formula mass
The unit for gram formula mass is grams/mole
 How many moles in 49 grams of H2SO4?
49 g
= X moles
98 g/mol
X= 0.50 moles
 How many grams are contained in 2.00 moles of H2SO4?
X
= 2.00 moles
98 g/mol
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X = 196 grams
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Types of chemical formulas
include empirical, molecular,
and structural.
Molecular
Formula
Empirical
Formula
Structural
Formula
Which elements
and how many of
each in a molecule
Which elements
and the ratio of
how many of each
Shows the bonds
and how the atoms
are arranged
C2H6
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CH3
H
H C
H
H
C H
H
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The empirical formula of a
compound is the simplest wholenumber ratio of atoms of the
elements in a compound. It may
be different from the molecular
formula, which is the actual ratio
of atoms in a molecule of that
compound.
Hydrogen peroxide
molecular formula: H2O2
empirical formula: HO
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Sugar
C6H12O6
CH2O
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Regents Question: 06/03 #8
Which is an empirical formula?
(1) P2O5
(2) P4O6
(3) C2H4
(4) C3H6
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The molecular formula must be a
whole number multiple of the
empirical formula
Molecular
Formula
H2O2
C3H6
C6H12O6
Hg2O2
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Empirical
Formula
HO
CH2
CH2O
HgO
Multiple
2
3
6
2
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The formula mass must be a
whole number multiple of the
empirical mass
Molecular
Formula Mass
H2O2
34
C3H6
42
C6H12O6 180
Hg2O2
434
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Empirical
Formula Mass
HO
17
CH2
14
CH2O
30
HgO
217
Multiple
2
3
6
2
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Regents Question: 06/03 #19
Which formula correctly represents the compo-sition
of iron (III) oxide?
(1) FeO3
(2) Fe2O3
(3) Fe3O
(4) Fe3O2
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The percent composition by
mass of each element in a
compound can be calculated
mathematically.
Calculate the formula mass and then divide the component
of the element you are looking for by the total mass of the
formula then multiply by 100%.
Formula mass of H2SO4
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H 2x1 = 2
S 1 x 32 = 32
O 4 x 16 = 64
98
% of Oxygen in H2SO4
64/98 x 100% = 65%
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Regents Question: 01/03 #8
What is the percent by mass of oxygen in H2SO4?
[ formula mass = 98]
(1) 16%
(2) 33%
64/98 x 100
(3) 65%
(4) 98%
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Regents Question: 06/02 #7
The percent by mass of hydrogen in NH3 is equal to
(1) 17 x 100
1
(2) 17 x 100
3
(3) 1
x 100
17
(4) 3
x 100
17
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Regents Question: 06/03 #10
The percent by mass of calcium in the compound
calcium sulfate (CaSO4 ) is approximately
(1) 15%
(2) 29%
(3) 34%
(4) 47%
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A hydrate is a compound which
has water trapped in its crystal
structure. We can determine the
percentage of water in a hydrate.
 CuSO45H2O – for every copper(II) sulfate there
are 5 trapped water molecules
 Find the formula mass including the water
–
–
–
–
–
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Cu
S
O
H
O
1x
1x
4x
10 x
5x
64 = 64
32 = 32
16 = 64
1 = 10
16 = 80
250
Water contributes 90 to
the 250
90/250 x 100% = 36%
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Regents Question: 06/02 #36
What is the total number of oxygen atoms in the formula
MgSO4•7H2O? [The • represents seven units of H2O
attached to one unit of MgSO4]
(1) 11
(2) 7
(3) 5
(4) 4
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Use the coefficients to predict the
amount of reactant consumed or
product formed
(Mole-Mole problems)
 The ratio of the coefficients is a ratio of the moles
taking part in a reaction.
 Reactants (on the left of the arrow) are consumed
 Products (on the right of the arrow) are formed
 Given the number of moles of any substance in a
reaction, you can use the coefficients to find the
number of moles of any other substance.
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Place the moles given over the
coefficient for that substance and
set up a proportion with the
coefficients.
 How many moles of oxygen are produced when
4 moles of KClO3 react according to the
equation: 2 KClO3
2 KCl + 3 O2
unknown
4
X
given
2 KClO3
2 KCl + 3 O2
4 = X
2
3 cross multiply to get 2X = 12
X=6 moles
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Regents Question: 01/03 #42
Given the equation:
2C2H2(g) + 5O2(g)
4 CO2(g) + 2H2O(g)
How many moles of oxygen are required to react
completely with 1.0 mole of C2H2?
(1) 2.5
(2) 2.0
(3) 5.0
(4) 10
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Regents Question: 06/02 #37
Given the reaction:
6CO2 +6H2O
C6H12O6 + 6O2
what is the total number of moles of water needed to make
2.5 moles of C6H12O6 ?
(1) 2.5
(2) 6.0
(3) 12
(4) 15
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Regents Question: 06/03 #20
Given the reaction:
PbCl2 (aq) + Na2CrO4 (aq)
2NaCl(aq)
PbCrO4 (s) +
What is the total number of moles of NaCl formed
when 2 moles of Na2CrO4 react completely?
(1) 1 mole
(2) 2 moles
(3) 3 moles
(4) 4 moles
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