Edward A' Mottel

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Isotopes, Moles and Molecules

• Edward A. Mottel
• Department of Chemistry
• Rose-Hulman Institute of Technology

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Resources Available

• Reading assignment
• Zumdahl Chapter 3.1-3.5

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Atoms are Small
Generally measured in atomic mass units (amu) or
daltons
Carbon-12 (6 protons, 6 neutrons and 6
electrons) has a mass of exactly 12.000 amu
Why is the mass of carbon listed as 12.01 and not
12.00?
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Isotopes of Carbon
Isotopes of carbon
mass (amu)
protons
neutrons
electrons
12.000000
natural abundance
12C 98.90
13C 1.10
14C 0.0000000000001
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Isotopes of Silver

• Silver has two common isotopes
• silver-107 and silver-109
• Silver-107 has 47 protons and 60 neutrons

Why are these atoms considered to be isotopes of
each other?
What is the same and different about each isotope?
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The average atomic mass of silver is
107.868.Calculate the mass of the heavier
isotope
106.905
51.82

• (mass 107Ag)( 107Ag)
• (mass 109Ag)( 109Ag) avg. atomic mass 100

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Solve Equation

• soln solve((106.9050.5182) m109Ag(1 -
  0.5182) 107.868, m109Ag)

soln 108.9037547
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Amadeo Avogadro (1811)
In a fixed volume, at a given temperature
and pressure, every gas contains the same number
of particles (molecules).
Avogadro never heard of the term mole, or
Avogadros Number!
Theory was not accepted for 50 years.
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A Mole is Big
The conversion factor from an atomic quantity
(amu) to a measurable quantity (gram) is the mole.
6.022 x 1023 amu 1.000 mole of amu 1.000 gram
One mole equals 6.022 x 1023 units of anything.
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A Mole is Big
How many atoms are in one gram of carbon?
How many atoms are in one gram of carbon dioxide?
Which has more atoms, one gram of carbon monoxide
or one gram of carbon dioxide?
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Chemical Analysis andEmpirical Formulas

• Percentage composition by mass of a compound
• Determination of the empirical formula from
  percentage by mass information
• Empirical versus molecular formulas

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Percentage Composition
What is the percentage silver in silver nitrate?

• Silver Nitrate AgNO3
• Molar Mass 1(AW Ag) 1(AW N) 3(AW O)
• ( Ag) 1 ((AW Ag)/(Molar Mass)) 100
• ( N) 1 ((AW N)/(Molar Mass)) 100
• ( O) 3 ((AW O)/(Molar Mass)) 100

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Click on the spreadsheet to get live calculations
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Empirical Formula
Empirical formula problems are the reverse of
percentage composition problems.
A compound that contains only carbon,
hydrogen and oxygen is 48.38 C and 8.12 H.
What is the empirical formula of this compound?
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Empirical Formula CnCHnHOnO(If you like
systems of linear equations and Maple)

• eqn1 mass nC12.01 nH1.008 nO16.00
• eqn2 nC12.01/mass 0.4838
• eqn3 nH1.008/mass 0.0812
• eqn4 nO16.00/mass (1 - 0.4838 - 0.0812)
• sln solve(eqn1, eqn2, eqn3, eqn4, mass, nC,
  nH, nO)
• sln nO .6749108619 nC, mass 24.82430757
  nC, nH 1.999735887 nC, nC nC
• (CH2O2/3)nC -----gt (C3H6O2) nU

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Empirical Formula CnCHnHOnO(If you like to
think in terms of masses)

• Assume 100 grams of unknown.
• Mass of carbon 48.38 g
• Moles of carbon 48.38 / 12.01 4.028
• Mass of hydrogen 8.12 g
• Moles of hydrogen 8.12 / 1.008 8.055
• Mass of oxygen 100.00 48.38 - 8.12 43.50 g
• Moles of oxygen 43.50 / 16.00 2.719

C4.028 H8.055 O2.719
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C4.028 H8.055 O2.719
C1.481 H2.962 O1.000
C2.963 H5.925 O2.000
C3 H6 O2
The empirical formula is the simplest whole
number ratio of elements in a compound.
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Empirical vs. Molecular Formulas

• Empirical Formula (C3H6O2) nU
• Possible Molecular Formulas
• C3H6O2 nU 1
• C6H12O4 nU 2
• C9H18O6 nU 3

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Carbohydrates
Carbohydrates are compounds with the general
formula Cm(H2O)n where m is typically 5 or
greater and nltm.
Show that sucrose, C12H22O11 , is a carbohydrate.
Do all carbohydrates have the same percentage
composition for each element?
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Compounds which contain only carbon and
hydrogen are called hydrocarbons.
A hydrocarbon is burned in oxygen and
produces 0.400 grams of water and 0.782 grams of
carbon dioxide.
What is the empirical formula of the hydrocarbon?
What was the original mass of the
hydrocarbon that was combusted?
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Problem Solving Format

• Define the problem
• Outline the solution to the problem
• include relevant diagrams equations
• Solve the problem

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A hydrocarbon is burned in oxygen and
produces 0.400 grams of water and 0.782 grams of
carbon dioxide.
mol H
0.400 g H2O
0.0444
mol C
0.782 g CO2
0.0178
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C0.0178 H0.0444
What is the empirical formula of the hydrocarbon?
C1.00 H2.49
C2H5
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0.0444 mol H
0.0178 mol C
What was the original mass of the
hydrocarbon that was combusted?
0.0444 mol H
0.0448
g H
0.0178 mol C
0.214
g C
0.259 g
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