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Chapter 5 Chemical Quantities and Reactions
5.7 Mole Relationships in
Chemical Equations
Copyright © 2009 by Pearson Education, Inc.
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Law of Conservation of Mass
The law of conservation of mass indicates that in an
ordinary chemical reaction, • matter cannot be created or destroyed.• no change in total mass occurs in a reaction.
• mass of products is equal to mass of reactants.
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Conservation of Mass
2 moles of Ag + 1 mole of S = 1 mole of Ag2S
2 (107.9 g) + 1(32.1 g) = 1 (247.9 g)
247.9 g reactants = 247.9 g product
Copyright © 2009 by Pearson Education, Inc
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Consider the following equation:4 Fe(s) + 3 O2(g) 2 Fe2O3(s)
This equation can be read in “moles” by placing theword “moles of” between each coefficient and formula.
4 moles of Fe + 3 moles of O2 2 moles of Fe2O3
Reading Equations in Moles
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A mole-mole factor is a ratio of the moles for any two
substances in an equation.
4Fe(s) + 3O2(g) 2Fe2O3(s)
Fe and O2 4 moles Fe and 3 moles O2
3 moles O2 4 moles Fe
Fe and Fe2O3 4 moles Fe and 2 moles Fe2O3
2 moles Fe2O3 4 moles Fe
O2 and Fe2O3 3 moles O2 and 2 moles Fe2O3
2 moles Fe2O3 3 moles O2
Writing Mole-Mole Factors
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Consider the following equation:
3H2(g) + N2(g) 2NH3(g)
A. A mole-mole factor for H2 and N2 is
1) 3 moles N2 2) 1 mole N2 3) 1 mole N2
1 mole H2 3 moles H2 2 moles H2
B. A mole-mole factor for NH3 and H2 is
1) 1 mole H2 2) 2 moles NH3 3) 3 moles N2
2 moles NH3 3 moles H2 2 moles NH3
Learning Check
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3H2(g) + N2(g) 2NH3(g)
A. A mole-mole factor for H2 and N2 is 2) 1 mole N2
3 moles H2
B. A mole-mole factor for NH3 and H2 is
2) 2 moles NH3
3 moles H2
Solution
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How many moles of Fe2O3 can form from 6.0 moles of O2?
4Fe(s) + 3O2(g) 2Fe2O3(s)
Relationship: 3 mole O2 = 2 mole Fe2O3
Use a mole-mole factor to determine the moles of Fe2O3.
6.0 mole O2 x 2 mole Fe2O3 = 4.0 moles of Fe2O3
3 mole O2
Calculations with Mole Factors
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Guide to Using Mole Factors
Copyright © 2009 by Pearson Education, Inc.
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How many moles of Fe are needed for the reaction of 12.0 moles of O2?
4Fe(s) + 3O2(g) 2Fe2O3(s)
1) 3.00 moles of Fe 2) 9.00 moles of Fe3) 16.0 moles of Fe
Learning Check
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In a problem, identify the compounds given and needed.
How many moles of Fe are needed for the reaction of12.0 moles of O2?
4Fe(s) + 3O2(g) 2Fe2O3(s)
The possible mole factors for the solution are
4 moles Fe and 3 moles O2
3 moles O2 4 moles Fe
Study Tip: Mole Factors
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3) 16.0 moles of Fe
12.0 moles O2 x 4 moles Fe = 16.0 moles of Fe 3 moles O2
Solution