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Chapter SixChapter SixCalculations:
Formula Masses, Moles, and
Chemical Equations
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Chapter 6 | Slide 2
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
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Chapter 6 | Slide 3
Examples
Molecular mass of H2O is18 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
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Chapter 6 | Slide 4
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
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Chapter 6 | Slide 5
How big is a mole?
1 mole = 6.02 x 1023 of anything1 mole of atoms is _______ atoms1 mole of baseballs is ______ baseballs1 mole of dimes is ______ dimes
If there were 1 mole of people, it
would take 120 trillion Earths to
accommodate them!
6.02 x 1023 = 1 mole
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Chapter 6 | Slide 6
← Fig. 6.3 Everyday counting units.
Calculations: Formula Masses, Moles, and Chemical Equations cont’d
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Chapter 6 | Slide 7
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
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Chapter 6 | Slide 8
Fig. 6.7In solving chemical-formula-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.
Calculations: Formula Masses, Moles, and Chemical Equations cont’d
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Chapter 6 | Slide 9
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?
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Chapter 6 | Slide 10
→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.
Calculations: Formula Masses, Moles, and Chemical Equations cont’d
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Chapter 6 | Slide 11
The Mass of a Mole
The mass of a mole is not a set number of grams; it depends on the substance.
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Chapter 6 | Slide 12
The Mass of a Mole
Consider: How much does a dozen weigh?
All depends on what dozen1 dozen pennies1 dozen eggs1 dozen bricks
The mass of a mole depends on the specific substance.
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Chapter 6 | Slide 13
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!
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Chapter 6 | Slide 14
Calculations Involving Molar Mass
How many grams are there in exactly three moles of NaCl?
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Chapter 6 | Slide 15
Calculations Involving Molar Mass
How many moles are in 215 g of silver chloride (AgCl)?
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Chapter 6 | Slide 16
Molar mass in grams
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Chapter 6 | Slide 17
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
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Chapter 6 | Slide 18
Chemical Formulas as Conversion Factors
Calculate the number of moles of hydrogen atoms in 42.11 mol of H2O.
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Chapter 6 | Slide 19
Chemical Formulas as Conversion Factors
Calculate the number of moles of Citronellal molecules, C10H18O, that contain 2.35 x 1025 moles of carbon atoms.
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Chapter 6 | Slide 20
Fig. 6.7In solving chemical-formula-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.
Calculations: Formula Masses, Moles, and Chemical Equations cont’d
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Chapter 6 | Slide 21
The Mole and Chemical Calculations
Calculate the number of molecules of carbon dioxide in 129.7 g of carbon dioxide.
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Chapter 6 | Slide 22
The Mole and Chemical Calculations
Calculate the mass of 4.14 x 1025 molecules of diethyl ether, C4H10O.
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Chapter 6 | Slide 23
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
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Chapter 6 | Slide 24
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.
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Chapter 6 | Slide 25
Chemical Equations are like Recipes!
Banana Chocolate Chip SoftiesThese 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 flour1 teaspoon baking powder1/2 teaspoon salt1/3 cup butter, softened1/4 cup light brown sugar1 ripe banana, mashed1 egg1 teaspoon vanilla extract3/4 cup milk chocolate chipsDirections1 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 ALL RIGHTS RESERVED Copyright © 2004 www.allrecipes.com
The Thermite Reaction glycerin
2Al(s) + Fe2O3(s) ----> Al2O3(l) + 2Fe(l) Reactants Products
Similarities:•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.
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Chapter 6 | Slide 26
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
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Chapter 6 | Slide 27
The Numbers in Chemical Equations
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Chapter 6 | Slide 28
Balanced Chemical Equations
Chemical Equations must be balancedThere 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.
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Chapter 6 | Slide 29
Practice: Balancing Chemical Equations
Solid sodium nitride decomposes to form solid sodium metal and nitrogen gas
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Chapter 6 | Slide 30
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.
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Chapter 6 | Slide 31
More Practice: Balancing Chemical Equations
NH3 + O2 N2 + H2O
Na2SO4 + C Na2S + CO2
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Chapter 6 | Slide 32
More Practice:Balancing Combustion Reactions
C2H6 + O2 CO2 + H2O
C3H6 + O2 CO2 + H2O
NH3 + O2 NO + H2O
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Chapter 6 | Slide 33
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.
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Chapter 6 | Slide 34
S’mores Stoichiometry
1 S’more
+
+
2 GC
3 Cp
1 Mm
What are the ratios?
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Chapter 6 | Slide 35
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!
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Chapter 6 | Slide 36
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
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Chapter 6 | Slide 37
Fig. 6.9 In solving chemical-equation-based problems, the only “transitions” allowed are those between quantities (boxes) connected by arrows.
Calculations: Formula Masses, Moles, and Chemical Equations cont’d
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Chapter 6 | Slide 38
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
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Chapter 6 | Slide 39
Practice with Mole Ratios
How many moles of O2 are required to completely react with 1.20 moles of C4H10?
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Chapter 6 | Slide 40
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.
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Chapter 6 | Slide 41
More Practice with Mole Ratios
How many moles of O2 react with 2.03 moles of CS2?
3O2 + CS2 CO2 + 2SO2
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Chapter 6 | Slide 42
More Practice with Mole Ratios
How many moles of SO2 are produced when 2.03 moles of CS2
react? 3O2 + CS2 CO2 + 2SO2
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Chapter 6 | Slide 43
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?
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Chapter 6 | Slide 44
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?
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Chapter 6 | Slide 45
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?
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Chapter 6 | Slide 46
Stoichiometry Calculations, Part IV
Automotive airbags inflate when sodium azide, NaN3, rapidly decomposes to its constituent elements. The equation for the chemical reaction is2NaN3 (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?