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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
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Chemical equations help us to describe chemical reactions.
BEFORE AFTER
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Combustion (of a hydrocarbon with O2) Combination Reactions Decomposition Reactions
Single Displacement Reactions (Ch.4) Double Displacement Reactions (Ch.4)
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Hydrocarbons react with oxygen to give carbon dioxide and water:
CH3CH2CH3(g) + O2(g) CO2(g) + H2O(g)
Balance it!
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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 Reactions: A + B C
Mg(s) + O2(g) ?
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Combination Reactions: A + B C
Mg(s) + O2(g) MgO(s)
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Combination Reactions: A + B C
2Mg(s) + O2(g) 2MgO(s)
Fig 3.5
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Decomposition Reactions: C A + B
Sodium azide decomposes to its elements:
NaN3(s) ?
(N3- is a polyatomic ion)
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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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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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Another example:
2HgO(s) 2Hg(l) + O2(g)
Rxn
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Some compounds decompose to more stable compounds:
2H2O2(l) 2H2O(l) + O2(g)
CaCO3(s) CaO(s) + CO2(g)
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Atomic Weight Molecular Weight / Formula Weight Percent Composition Empirical Formulas
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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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Atomic weight is different than mass number.
Isotope Mass
NumberAtomicMass
Abundance
Cu-63 63 62.94 amu 69.17 %
Cu-65 65 64.93 amu 30.83 %
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(62.94 amu)(0.6917) = 43.5356(64.93 amu)(0.3083) = 20.0179
63.55 amu (4 sig figs)
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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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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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Mass Percent Composition =
(#Atoms of Element)(Atomic Weight) x100 Formula Weight of Compound
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%46.74100amu21.95
amu90.70x
What is the mass %Cl in MgClWhat is the mass %Cl in MgCl22??Formula weight = (2 x 35.45amu) + 24.31 amuFormula weight = (2 x 35.45amu) + 24.31 amu
95.21 amu95.21 amu
Mass of Cl: 2 x 35.45amu = 70.90amuMass of Cl: 2 x 35.45amu = 70.90amu
Mass %Cl = Mass %Cl =
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What is the mass %Mg in MgCl2?
74.47% Cl 25.53% Mg
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How would you go about determining the mass of carbon in 5.0 g CCl4?
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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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Which of the following has the greatest %O? Explain.
MgO CaO Na2O
K2O
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What is the percent carbon in C2H4?
What is the percent carbon in C3H6?
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What is the percent carbon in C2H4? % C = 85.63%
What is the percent carbon in C3H6? % C = 85.63%
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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.
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Which of the following are empirical formulas?
CH4
C2H2
N2O5
C2H6
Al2O3
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Which of the following are empirical formulas?
CH4
C2H2
N2O5
C2H6
Al2O3