1. Chapter Three:
STOICHIOMETRY

2. Chemical Stoichiometry
Stoichiometry – The study of quantities of materials consumed and produced in chemical reactions.
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3. Counting by Weighing Need average mass of the object.
Objects behave as though they were all identical.
3.1
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4. Atomic Masses Elements occur in nature as mixtures of isotopes.
Carbon = % 12C
1.11% 13C
<0.01% 14C
Carbon atomic mass = amu
3.2
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5. Schematic Diagram of a Mass Spectrometer
3.2
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6. The Mole
The number equal to the number of carbon atoms in exactly 12 grams of pure 12C
1 mole of anything = x 1023 units of that thing (Avogadro’s number)
1 mole C = x 1023 C atoms = g C
3.3
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7. Molar Mass Mass in grams of one mole of the substance:
Molar Mass of N = g/mol
Molar Mass of H2O = g/mol
(2 × 1.008)
Molar Mass of Ba(NO3)2 = g/mol
(2 × 14.01) + (6 × 16.00)
3.4
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8. Concept Check
Which of the following is closest to the average mass of one atom of copper?
a) g
b) g
c) g
d) g
e) x g
The correct answer is “e”. The mass of one atom of copper is going to be extremely small, so only letter “e” makes sense. The calculated solution is (1 Cu atom) × (1 mol Cu/6.022×1023 Cu atoms) × (63.55 g Cu/1 mol Cu).
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9. Concept Check
Calculate the number of copper atoms in a g sample of copper.
6.022×1023 Cu atoms; g of Cu is 1 mole of Cu, which is equal to Avogadro’s number.
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10. Concept Check
Which of the following g samples contains the greatest number of atoms?
Magnesium
Zinc
Silver
The correct answer is “a”. Magnesium has the smallest average atomic mass (24.31 amu). Since magnesium is lighter than zinc and silver, it will take more atoms to reach g.
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11. Exercise
Rank the following according to number of atoms (greatest to least):
107.9 g of silver
70.0 g of zinc
21.0 g of magnesium
b, a, c; The greater the number of moles, the greater the number of atoms present. Zinc contains 1.07 mol, Ag contains mol, and Mg contains mol.
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12. Exercise
Consider separate gram samples of each of the following: 
H2O, N2O, C3H6O2, CO2
Rank them from greatest to least number of oxygen atoms.
H2O, CO2, C3H6O2, N2O; The number of oxygen atoms in each is:
H2O = 3.343×1024
CO2 = 2.737×1024
C3H6O2 = 1.626×1024
N2O = 1.368×1024
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13. Percent Composition Mass percent of an element:
For iron in iron(III) oxide, (Fe2O3):
3.5
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14. Exercise
Consider separate gram samples of each of the following: 
H2O, N2O, C3H6O2, CO2
Rank them from highest to lowest percent oxygen by mass.
H2O, CO2, C3H6O2, N2O; The percent oxygen by mass in each is:
H2O = 88.81%
CO2 = 72.71%
C3H6O2 = 43.20%
N2O = 36.35%
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15. Formulas Molecular formula = (empirical formula)n [n = integer]
Molecular formula = C6H6 = (CH)6
Actual formula of the compound
Empirical formula = CH
Simplest whole-number ratio
3.6
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16. Analyzing for Carbon and Hydrogen
Device used to determine the mass percent of each element in a compound.
3.6
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17. Balancing Chemical Equations
3.7 and 3.8
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18. Chemical Equation A representation of a chemical reaction:
C2H5OH + 3O2  2CO H2O
reactants products
Reactants are only placed on the left side of the arrow, products are only placed on the right side of the arrow.
3.7 and 3.8
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19. Chemical Equation C2H5OH + 3O2  2CO2 + 3H2O The equation is balanced.
All atoms present in the reactants are accounted for in the products.
1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water.
3.7 and 3.8
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20. Chemical Equations
The coefficients in the balanced equation have nothing to do with the amount of each reactant that is given in the problem.
3.7 and 3.8
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21. Chemical Equations
The balanced equation represents a ratio of reactants and products, not what actually “happens” during a reaction.
Use the coefficients in the balanced equation to decide the amount of each reactant that is used, and the amount of each product that is formed.
3.7 and 3.8
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22. Exercise
Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it.
CaO + C  CaC2 + CO2
I. CaO2 + 3C  CaC2 + CO2
II. 2CaO + 5C  2CaC2 + CO2
III. CaO + (2.5)C  CaC2 + (0.5)CO2
IV. 4CaO + 10C  4CaC2 + 2CO2
II, III, and IV are correct. I is not correct because you cannot use subscripts to balance a chemical equation.
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23. Concept Check
Which of the following are true concerning balanced chemical equations? There may be more than one true statement.
I. The number of molecules is conserved.
II. The coefficients tell you how much of each substance you have.
III. Atoms are neither created nor destroyed.
IV. The coefficients indicate the mass ratios of the substances used.
V. The sum of the coefficients on the reactant side equals the sum of the coefficients on the product side.
Only III is correct.
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24. Notice
The number of atoms of each type of element must be the same on both sides of a balanced equation.
Subscripts must not be changed to balance an equation.
A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction.
Coefficients can be fractions, although they are usually given as lowest integer multiples.
3.8
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25. Stoichiometric Calculations
Chemical equations can be used to relate the masses of reacting chemicals.
3.9
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26. Calculating Masses in Reactions
Balance the equation.
Convert mass to moles.
Set up mole ratios from the balanced equation.
Calculate number of moles of desired reactant or product.
Convert back to grams.
3.9
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27. Exercise
Methane (CH4) reacts with the oxygen in the air to produce carbon dioxide and water.
Ammonia (NH3) reacts with the oxygen in the air to produce nitrogen monoxide and water.
What mass of ammonia would produce the same amount of water as 1.00 g of methane reacting with excess oxygen?
1.42 g of ammonia is required. Set up balanced equations for each reaction. Determine the amount of water produced from 1.00 g of methane (which is moles of water). Use the moles of water and the balanced equation to determine the amount of ammonia needed.
See the “Let’s Think About It” slide next to get the students going.
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28. Let’s Think About It We need to know:
How much water is produced from 1.00 g of methane and excess oxygen.
How much ammonia is needed to produce the amount of water calculated above.
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29. Limiting Reactants
Limiting reactant – the reactant that is consumed first and therefore limits the amounts of products that can be formed.
Determine which reactant is limiting to calculate correctly the amounts of products that will be formed.
3.10
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30. Limiting Reactants
3.10
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31. Limiting Reactants
Methane and water will react to form products according to the equation:
CH4 + H2O  3H2 + CO
3.10
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32. Mixture of CH4 and H2O Molecules Reacting
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33. CH4 and H2O Reacting to Form H2 and CO
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34. Limiting Reactants
The amount of products that can form is limited by the methane.
Methane is the limiting reactant.
Water is in excess.
3.10
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35. Concept Check
Which of the following reaction mixtures could produce the greatest amount of product? Each involves the reaction symbolized by the equation:
2H2 + O2  2H2O
a) 2 moles of H2 and 2 moles of O2
2 moles of H2 and 3 moles of O2
2 moles of H2 and 1 mole of O2
d) 3 moles of H2 and 1 mole of O2
e) Each produce the same amount of product
The correct answer is “e”. All of these reaction mixtures would produce two moles of water.
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36. Notice
We cannot simply add the total moles of all the reactants to decide which reactant mixture makes the most product. We must always think about how much product can be formed by using what we are given, and the ratio in the balanced equation.
3.10
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37. Concept Check
You know that chemical A reacts with chemical B. You react 10.0 g of A with 10.0 g of B.
What information do you need to know in order to determine the mass of product that will be produced?
See the “Let’s Think About It” slide next to get the students going.
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38. Let’s Think About It We need to know:
The mole ratio between A, B, and the product they form. In other words, we need to know the balanced reaction equation.
The molar masses of A, B, and the product they form.
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39. Exercise
You react 10.0 g of A with 10.0 g of B. What mass of product will be produced given that the molar mass of A is 10.0 g/mol, B is
20.0 g/mol, and C is 25.0 g/mol? They react according to the equation:
A + 3B  2C
8.33 g of C is produced.
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40. Percent Yield
An important indicator of the efficiency of a particular laboratory or industrial reaction.
3.10
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