Stoichiometry, Percentage Composition, Empirical Formulas

1
Stoichiometry,Percentage Composition, Empirical
Formulas

• E. A. Mottel (with modifications by JEF)
• Integrated First-Year Curriculum in Science,
  Engineering and Mathematics

2
Topics

• Isotopes - Percentage Composition
• Molecules - Percentage Composition
• Molecules - Empirical Formulas

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Isotope Composition

• Isotope1mass Isotope1percentage
• Isotope2mass Isotope2percentage
• Isotope3mass Isotope3percentage
• Atomic Mass

4
The element silver (Ag) has two naturally
occurring isotopes, 107 and 109. Its average
atomic mass is 107.868. Calculate the mass of
the heavier isotope

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

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Solve Equation

• sln solve(
• m109Ag(1 - 0.5182) 106.9050.5182
• 107.868,
• m109Ag)
• sln 108.9037547

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Chemical Analysis and Empirical 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

• Find the percentage composition by mass of
  phosphorus in Na3PO4.
• (mass P)
• (mass P) (mass P)/(molar mass)
• mass P 130.97
• molar mass 322.99 130.97 416
• (mass P) 0.1889

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Percentage Composition

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

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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

• eqn1 mmass nC12.01nO16.00nH1.008
• eqn2 nC12.01/mmass 0.4838
• eqn3 nO16.00/mmass (1 - 0.4838 - 0.0812)
• eqn4 nH1.008/mmass 0.0812
• sln solve(eqn1, eqn2, eqn3, eqn4, mmass,
  nH, nC, 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 vs. Molecular Formulas

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