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Chapter 12 Stoichiometry
12.1 The Arithmetic of Equations
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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In an ordinary chemical reaction, • Matter cannot be created nor destroyed.• The number of atoms of each element are equal. • The mass of reactants equals the mass of products.
H2(g) + Cl2(g) 2HCl(g)
2 mol H (1mol H2), 2 mol Cl (1mol Cl2) = 2 mol H, 2 mol Cl (2mol HCl)
2(1.008) + 2(35.45) = 2(36.46) 72.92 g = 72.92 g
Law of Conservation of Mass
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4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)four molecules NH3 react with five molecules O2
to produce
four molecules NO and six molecules H2O
and
four mol NH3 react with five mol O2
to produce
four mol NO and six mol H2O
Quantities in A Chemical Reaction
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Balance each equation. Write down the quantities (MOLES) of reactants and products:
1. Na + MgF2 →→NaF + Mg
2. Mg + HCl → MgCl2 + H2
3. Cl2 + KI → KCl + I2
4. NaCl→ Na + Cl2
5. Na + O2 → Na2O
6. Na + HCl → H2 + NaCl
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Conservation of Mass
2 mol Ag + 1 mol S = 1 mol Ag2S
2 (107.9 g) + 1(32.07 g) = 1 (247.9 g)
247.9 g reactants = 247.9 g product
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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Balance each equation. Calculate the masses of reactants and products:
1. Na + MgF2 →→NaF + Mg
2. Mg + HCl → MgCl2 + H2
3. Cl2 + KI → KCl + I2
4. NaCl→ Na + Cl2
5. Na + O2 → Na2O
6. Na + HCl → H2 + NaCl
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• The equation can be read in “moles” by placing the word “mole” or “mol” after each coefficient.
4Fe(s) + 3O2(g) 2Fe2O3(s)
4 mol Fe + 3 mol O2 2 mol Fe2O3
Moles in Equations
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A mole-mole factor is a ratio of the moles for two
substances in an equation.
4Fe(s) + 3O2(g) 2Fe2O3(s)
Fe and O2 4 mol Fe and 3 mol O2
3 mol O2 4 mol Fe
Fe and Fe2O3 4 mol Fe and 2 mol Fe2O3
2 mol Fe2O3 4 mol Fe
O2 and Fe2O3 3 mol O2 and 2 mol Fe2O3
2 mol Fe2O3 3 mol O2
Writing Mole-Mole Factors
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Balance each equation. Write down the mole factors possible:
1. Na + MgF2 →→NaF + Mg
2. Mg + HCl → MgCl2 + H2
3. Cl2 + KI → KCl + I2
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Consider the following equation:
3H2(g) + N2(g) 2NH3(g)
A. A mole factor for H2 and N2 is
1) 3 mol N2 2) 1 mol N2 3) 1 mol N2
1 mol H2 3 mol H2 2 mol H2
B. A mole factor for NH3 and H2 is
1) 1 mol H2 2) 2 mol NH3 3) 3 mol N2
2 mol NH3 3 mol H2 2 mol NH3
Learning Check
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3H2(g) + N2(g) 2NH3(g)
A. A mole factor for H2 and N2 is 2) 1 mol N2
3 mol H2
B. A mole factor for NH3 and H2 is
2) 2 mol NH3
3 mol H2
Solution
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Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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How many moles of Fe2O3 can form from 6.0 mol O2?
4Fe(s) + 3O2(g) 2Fe2O3(s)
STEP 1 Given 6.0 mol O2 Need: moles of Fe2O3.
STEP 2 moles O2 moles Fe2O3
STEP 3 3 mol O2 = 2 mol Fe2O3
3 mol O2 and 2 mol Fe2O3
2 mol Fe2O3 3 mol O2
STEP 4 Set up problem using the mol factor.
6.0 mol O2 x 2 mol Fe2O3 = 4.0 mol Fe2O3
3 mol O2
Calculations with Mole Factors
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How many moles of Fe are needed to react with
12.0 mol O2?
4Fe(s) + 3O2(g) 2 Fe2O3(s)
1) 3.00 mol Fe 2) 9.00 mol Fe3) 16.0 mol Fe
Learning Check
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3) 16.0 mol Fe
Consider the following reaction:
4Fe(s) + 3O2(g) 2 Fe2O3(s)
How many moles of Fe are needed to react with 12.0 mol O2?
12.0 mol O2 x 4 mol Fe = 16.0 mol Fe 3 mol O2
Solution
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Calculations with mole factors: 1. How many moles of Na are needed for 4 moles Mg? Na + MgF2 →→NaF + Mg
2. How many moles of HCl are needed for 2.5 moles of H2?
Mg + HCl → MgCl2 + H2
3. How many moles of KCl are formed from 5.8 moles KI?
Cl2 + KI → KCl + I2
4. How many moles of Na and Cl2 are formed when 6.5 moles NaCl decomposes?
NaCl→ Na + Cl2
5. How many moles of Na and O2 need to react in order to form 11 moles Na2O?
Na + O2 → Na2O
