Stoichiometry: Calculations with Chemical Formulas and Equations

1
StoichiometryCalculations with Chemical
Formulas and Equations

• Chapter 3

2
Chapter 3

• Chemical Equations
• Balancing, Types of Equations
• Compound Composition
• Atom mass Atomic and formula weight
• Composition
• Empirical Formulas
• Mole Amounts Molar Mass
• Mole Calculations
• Amounts in chemical reactions

3
Chemical Equations

• Chemical equations help us to describe chemical
  reactions.

BEFORE
AFTER
4
Types of Reactions(More in Ch.4)

• Combustion (of a hydrocarbon with O2)
• Combination Reactions
• Decomposition Reactions
• Single Displacement Reactions (Ch.4)
• Double Displacement Reactions (Ch.4)

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Combustion Reactionsin Air

• Hydrocarbons react with oxygen to give carbon
  dioxide and water
• CH3CH2CH3(g) O2(g) ?? CO2(g) H2O(g)
• Balance it!

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Combustion Reactionsin Air

• Hydrocarbons react with oxygen to give carbon
  dioxide and water
• C3H8(g) 5O2(g) ?? 3CO2(g) 4H2O(g)
• Balance it!

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Combination and Decomposition Reactions
Combination Reactions A B ? C Mg(s)
O2(g) ? ?
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Combination and Decomposition Reactions
Combination Reactions A B ? C Mg(s)
O2(g) ? MgO(s)
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Combination and Decomposition Reactions
Combination Reactions A B ? C 2Mg(s)
O2(g) ? 2MgO(s)
Fig 3.5
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Combination and Decomposition Reactions
Decomposition Reactions C ? A B Sodium
azide decomposes to its elements NaN3(s) ?
? (N3- is a polyatomic ion)
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Combination and Decomposition Reactions
Decomposition Reactions C ? A B Sodium
azide decomposes to its elements NaN3(s) ?
Na(s) N2(g) (N3- is a polyatomic
ion) Balance it!
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Combination and Decomposition Reactions
Decomposition Reactions C ? A B Sodium
azide decomposes to its elements 2NaN3(s) ?
2Na(s) 3N2(g) (N3- is a polyatomic ion)
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Decomposition Reactions

• Another example
• 2HgO(s) ? 2Hg(l) O2(g)

Rxn
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Decomposition Reactions

• Some compounds decompose to more stable
  compounds
• 2H2O2(l) ? 2H2O(l) O2(g)
• CaCO3(s) ? CaO(s) CO2(g)

15
Chemical CompositionThe quantitative
significance of formulas

• Atomic Weight
• Molecular Weight / Formula Weight
• Percent Composition
• Empirical Formulas

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What is the Mass of an Atom?

• When we describe the mass of atoms of an element,
  we must consider all of the naturally occurring
  isotopes.
• Carbon is composed of
• 98.8925 C-12 (12.00000 amu)
• 1.1080 C-13 (13.00335 amu)
• Its weighted average is 12.011 amu.
• Its atomic weight is 12.011 amu.

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What is the Atomic Weight of Copper?

• Atomic weight is different than mass number.

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What is the Average Atomic Weight of Copper?

• (62.94 amu)(0.6917) 43.5356
• (64.93 amu)(0.3083) 20.0179
• 63.55 amu (4 sig figs)

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Formula Weight

• The formula weight of a substance is the sum of
  the atomic weights of the atoms that make up the
  formula.
• Mg(OH)2 2(16.00) amu
• 2(1.008) amu
• 24.31 amu
• 58.33 amu formula wt. Mg(OH)2

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Molecular Weight

• The formula weight of a molecular compound is
  also called its molecular weight.
• H2O 1.008 amu
• 1.008 amu
• 16.00 amu
• 18.02 amu molecular wt. of H2O

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Percent Composition

• Mass Percent Composition
• (Atoms of Element)(Atomic Weight) x100
• Formula Weight of Compound

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Percent Composition

• What is the mass Cl in MgCl2?
• Formula weight (2 x 35.45amu) 24.31 amu
• 95.21 amu
• Mass of Cl 2 x 35.45amu 70.90amu
• Mass Cl

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Percent Composition

• What is the mass Mg in MgCl2?
• 74.47 Cl
• 25.53 Mg

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Group Work

• How would you go about determining the mass of
  carbon in 5.0 g CCl4?

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Group Work

• How would you go about determining the mass of
  carbon in 5.0 g CCl4?
• C (12.011amu/153.811 amu)x100
• 7.8089 C
• g Carbon (0.078089)(5.0g) 0.39 g

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Group Work

• Which of the following has the greatest O?
  Explain.
• MgO
• CaO
• Na2O
• K2O

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Composition

• What is the percent carbon in C2H4?
• What is the percent carbon in C3H6?

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Composition

• What is the percent carbon in C2H4?
• ? C 85.63
• What is the percent carbon in C3H6?
• ? C 85.63

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Empirical Formula

• Different compounds with the same percent
  composition have the same empirical formula.
• The empirical formula shows the ratio of atoms of
  each element as the smallest whole-number ratio.

30
What is the empirical formula for Benzene (C6H6)?
31
Group Work

• Which of the following are empirical formulas?
• CH4
• C2H2
• N2O5
• C2H6
• Al2O3

32
Group Work

• Which of the following are empirical formulas?
• CH4
• C2H2
• N2O5
• C2H6
• Al2O3