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Chapter Six
Calculations:
Formula Masses,
Moles, and
Chemical Equations
Molecular and Formula Mass
 Atomic mass unit (amu) = the unit of mass for
atoms
 Molecular mass is the ___ of the ______ of all
atoms in a ________
 Formula mass is the ___ of the ______ of all
atoms in a _______ _____ of an _____ compound
Chapter 6 | Slide 2
Examples
 Molecular mass of H2O is
18 amu
 Formula mass of Ca(NO3)2 is
1 Ca atom = 1 x 40.1 amu
2 N atoms = 2 x 14.0 amu
6 O atoms = 6 x 16.0 amu
Sum is 161.4 amu
Chapter 6 | Slide 3
Avogadro’s Number
 6.02 x 1023
 Avogadro’s number is equal
to __ ______
 Makes working with large numbers easier
 Instead of saying you have
12,000,000,000,000,000,000,000,000 molecules of NH3, you
can say you have 19.9 ______ of NH3
Chapter 6 | Slide 4
How big is a mole?
 1 mole = 6.02 x 1023 of anything
 1 mole of atoms is _______ atoms
 1 mole of baseballs is ______ baseballs
 1 mole of dimes is ______ dimes
 If there were 1 mole of people, it
would take 120 trillion Earths to
accommodate them!
6.02 x
23
10 = 1 mole
Chapter 6 | Slide 5
Calculations: Formula Masses, Moles,
and Chemical Equations cont’d
← Fig. 6.3
Everyday counting
units.
Chapter 6 | Slide 6
How big is a mole?
 The mole is a term of convenience
1 mole = 6.02 X 1023
1 dozen = 12
1 gross = 144 or 12 dozen
1 ream paper = 500 sheets of paper
1 ton (mass) = 2000 pounds
Chapter 6 | Slide 7
Calculations: Formula Masses, Moles,
and Chemical Equations cont’d
Fig. 6.7
In solving chemical-formula-based problems, the only
“transitions” allowed are those between quantities (boxes)
connected by arrows.
Chapter 6 | Slide 8
Calculations with Avogadro’s Number
 How many CO2 molecules are there in 2.3 moles
of CO2?
 A Tums tablet has 7.224 x 1023 atoms of Ca.
How many moles is this?
Chapter 6 | Slide 9
Calculations: Formula Masses, Moles,
and Chemical Equations cont’d
→ Fig. 6.2
A basic process in
chemical laboratory
work is determining
the mass of a
substance. We
don’t count or
measure out moles
directly.
Chapter 6 | Slide 10
The Mass of a Mole
The mass of a mole is not a set number of grams; it depends on
the substance.
Chapter 6 | Slide 11
The Mass of a Mole
Consider: How much does a dozen weigh?
All depends on what dozen
1 dozen pennies
1 dozen eggs
1 dozen bricks
The mass of a mole depends on the specific
substance.
Chapter 6 | Slide 12
The Mass of a Mole
 Molar mass is the mass of one mole of molecules, atoms,
ions, or formula units
 Numerically equal to the substance’s formula mass
 The only difference is the units:
►Molecular weight: weight of a molecule, in amu
►Molar mass: mass of one mole of a compound, in grams
 Calculated the same way
 Example: What is the molar mass of C8H9O2N?
Molar mass is a conversion factor between mass and moles!
Chapter 6 | Slide 13
Calculations Involving Molar Mass
 How many grams are there in exactly three moles
of NaCl?
Chapter 6 | Slide 14
Calculations Involving Molar Mass
 How many moles are in 215 g of silver chloride
(AgCl)?
Chapter 6 | Slide 15
Molar mass in grams
Chapter 6 | Slide 16
Chemical Formulas
as Conversion Factors
 The subscripts in a chemical formula tell you how many of each
kind of atom are in one molecule
 The subscripts also tell you how many moles of atoms of the
various elements are present in 1 mole of the substance
 Example: N2O4
Chapter 6 | Slide 17
Chemical Formulas
as Conversion Factors
 Calculate the number of moles of hydrogen atoms
in 42.11 mol of H2O.
Chapter 6 | Slide 18
Chemical Formulas
as Conversion Factors
 Calculate the number of moles of Citronellal
molecules, C10H18O, that contain 2.35 x 1025
moles of carbon atoms.
Chapter 6 | Slide 19
Calculations: Formula Masses, Moles,
and Chemical Equations cont’d
Fig. 6.7
In solving chemical-formula-based problems, the only
“transitions” allowed are those between quantities (boxes)
connected by arrows.
Chapter 6 | Slide 20
The Mole and Chemical Calculations
 Calculate the number of molecules of carbon
dioxide in 129.7 g of carbon dioxide.
Chapter 6 | Slide 21
The Mole and Chemical Calculations
 Calculate the mass of 4.14 x 1025 molecules of
diethyl ether, C4H10O.
Chapter 6 | Slide 22
Chemical Equations
•Lavoisier:
mass is conserved in a
chemical reaction.
•Chemical equations:
symbolic descriptions of
chemical reactions.
•Two parts to an equation:
•reactants and
•products:
2H2 + O2  2H2O
Chapter 6 | Slide 23
Mass is Conserved
During a Chemical Reaction
When 16.90 g of compound CaS (left) is decomposed into its
constituent elements the Ca and S produced (right) has an identical
mass of 16.90 grams.
Chapter 6 | Slide 24
Chemical Equations are like Recipes!
Similarities:
Banana Chocolate Chip Softies
These cookies are nice change of pace from the regular chocolate chip
cookie. They have extra sweetness form the ripe banana and milk chocolate
chips. Prep Time: approx. 8 Minutes. Cook Time: approx. 11 Minutes. Ready in:
approx. 20 Minutes. Makes 2 dozen (24 servings).
Printed from Allrecipes, Submitted by Kim 1 1/4 cups all-purpose flour
1 teaspoon baking powder
1/2 teaspoon salt
1/3 cup butter, softened
1/4 cup light brown sugar1 ripe banana, mashed
1 egg
1 teaspoon vanilla extract
3/4 cup milk chocolate chips
Directions
1 Preheat oven to 350 degrees F (175 degrees C). Grease cookie sheets. Sift together the flour,
baking powder and salt, set aside.
2 In a medium bowl, cream together the butter and brown sugar. Beat in the banana and egg, then
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www.allrecipes.com
•Name.
•What ingredients are added
together.
•What is made.
•The physical state of ingredients.
•How much of each ingredient is
needed.
•How many products are made.
•Conditions needed for the reaction
to occur.
The Thermite Reaction
glycerin
2Al(s) + Fe2O3(s)
---->
Al2O3(l) + 2Fe(l)
Reactants
Products
Chapter 6 | Slide 25
Chemical Equations:
The Numbers
 Coefficient: numbers in front of the chemical formulas;
give ratio of reactants and products. CONVERSION
FACTORS!!!
►Changing these changes the number of molecules
 Formula subscript: number in the middle of chemical
formulas; give the number of each kind of atoms in
individual molecules.
►Changing these changes the identity of the compound

2Na + Cl2  2NaCl
Chapter 6 | Slide 26
The Numbers
in Chemical Equations
Chapter 6 | Slide 27
Balanced Chemical Equations
 Chemical Equations must be balanced
There must be equal numbers of atoms of each element
on both sides of the equation (both sides of the arrow)
►1. Write the correct symbols and formulas for all of the _______
and ____________.
►2. Count the number of each type of _____ on BOTH sides of
the __________.
►3. Insert ____________ (numbers to the left of the compound
formulas) until there are the equal numbers of each kind of
_______ on both sides of the equation.
Chapter 6 | Slide 28
Practice:
Balancing Chemical Equations
 Solid sodium nitride decomposes to form
solid sodium metal and nitrogen gas
Chapter 6 | Slide 29
Hints on Balancing Equations
 1. When there is no coefficient written, the




coefficient is assumed to be 1
2. To balance the equation insert COEFFICIENTS.
3. NEVER alter the subscripts because that would
change the chemical formula which would change
the identity and properties of the substance.
4. Start with atoms that only show up in only one
compound on each side of the equation arrow.
5. The only way to learn to balance equations is
through practice.
Chapter 6 | Slide 30
More Practice:
Balancing Chemical Equations
NH3 +
Na2SO4 +
O2 
C
N2 +
H2O
Na2S +
CO2
Chapter 6 | Slide 31
More Practice:
Balancing Combustion Reactions
C2H6 +
C3H6 +
NH3 +
O2 
O2 
O2 
CO2 +
H2O
CO2 +
H2O
NO +
H2O
Chapter 6 | Slide 32
Chemical Equations as Conversion
Factors: Mole Ratios
 The coefficients in an equation may be used to generate conversion
factors used in problem solving. These conversion factors are called
“mole ratios.”
 4Fe (s) + 3O2 (g)  2Fe2O3 (s)
 MOLE RATIOS (from equation coefficients) ARE CONVERSION
FACTORS BETWEEN: number of moles of one compound and
number of moles of another compound.
Chapter 6 | Slide 33
S’mores Stoichiometry
What are the ratios?
+
2 GC
1 S’more
+
1 Mm
3 Cp
Chapter 6 | Slide 34
Stoichiometry
 Chemical Stoichiometry: using mass and quantity relationships
among reactants and products in a chemical reaction to make
predictions about how much of a product will be made.
 Example: Let’s say that you have lots of marshmallows and chocolate
but only 6 graham crackers. How many s’mores can you make?
 Use the mole ratios from the equations!
Chapter 6 | Slide 35
Conversion Factors:
A Reminder
 Molar Mass:
1 mole X = # g X
 Avogadro’s Number: 1 mole = 6.02 X 1023
 Mole Ratios: from coefficients found in chemical
equations
2 H2 + O2  2 H2O
2 moles H2 = 1 mole O2 = 2 moles H2O
Chapter 6 | Slide 36
Calculations: Formula Masses, Moles,
and Chemical Equations cont’d
Fig. 6.9
In solving chemical-equation-based problems, the only
“transitions” allowed are those between quantities (boxes)
connected by arrows.
Chapter 6 | Slide 37
Practice with Mole Ratios
Example: How many moles of CO2 can be produced
from 1.20 moles of C4H10 in the following reaction?
 2 C4H10 + 13O2  8CO2 + 10H2O
Chapter 6 | Slide 38
Practice with Mole Ratios
How many moles of O2 are required to
completely react with 1.20 moles of C4H10?
Chapter 6 | Slide 39
Practice with Mole Ratios
A can of butane lighter fluid contains 1.20 moles of
butane (C4H10). Calculate the number of moles of
carbon dioxide given off when this butane is burned.
Chapter 6 | Slide 40
More Practice with Mole Ratios
How many moles of O2 react with 2.03 moles of CS2?
3O2 + CS2  CO2 + 2SO2
Chapter 6 | Slide 41
More Practice with Mole Ratios
How many moles of SO2 are produced when 2.03 moles of CS2
react? 3O2 + CS2  CO2 + 2SO2
Chapter 6 | Slide 42
Stoichiometry Calculations, Part I
1. Some sulfur is present in coal in the form of pyrite (FeS2, also known as “fool’s
gold”). When it burns, it pollutes the air with SO2, as follows:
FeS2 (s) + O2 (g)  Fe2O3 (s) + SO2 (g)
What mass of SO2 is produced by the combustion of 38.8 g of FeS2?
Chapter 6 | Slide 43
Stoichiometry Calculations, Part II
FeS2 (s) + O2 (g)  Fe2O3 (s) + SO2 (g)
How many grams of O2 are needed to react with 38.8 g of FeS2?
Chapter 6 | Slide 44
Stoichiometry Calculations, Part III
Over the years, the thermite reaction has been used for welding railroad rails, in
incendiary bombs, and to ignite solid-fuel rocket motors. The reaction is:
Fe2O3(s) + 2Al(s) -> 2Fe(l) + Al2O3(s)
What masses of iron(III) oxide and aluminum must be used to produce 15.0 g
iron?
Chapter 6 | Slide 45
Stoichiometry Calculations, Part IV
Automotive airbags inflate when sodium azide, NaN3, rapidly decomposes to
its constituent elements. The equation for the chemical reaction is
2NaN3 (s)  2Na (s) + 3N2 (g)
The gaseous N2 so generate inflates the airbag. How many moles of NaN3
would have to decompose in order to generate 2.53 x 108 molecules of
N2?
Chapter 6 | Slide 46