Zumdahls Chapter 3

1
Zumdahls Chapter 3

• Stoichiometry

2
Chapter Contents

• Astons Atomic Mass
• The Mole
• Molar Mass
• Composition
• Molecular Formulae
• Chemical Equations

• Balancing Chemical Equations
• Stoichiometric Calculations
• Limiting Reactant Calculations

3
Astons Atomic Masses(works for molecules too!)

• Mass Spectrometer
• Fast atomic ions (current) bend near magnet
• Deflection varies inversely with inertia!
• Multiple isotopes differ in mass (inertia) and so
  give multiple beam deflections.
• 98.892 12C at 12 amu by definition.
• 1.108 13C at 13.00338 amu
• averageC ? 12.011 atomic mass units

4
Avogadros Mole

• Definition One mole of 12C atoms weighs exactly
  0.012 kg (12 g)
• Thus, 1 amu ? 1 g / NAv
• Since atoms combine by numbers, NAv has the
  advantage of showing combinations by weights.
• NAv 6.0221367?1023, the SI count

5
Molar Masses Composition

• Trivial weigh them in mass spectrometer?
• Since atomic masses of elements never vary, MW
  sum of atomic weights times number of atoms in
  molecule (subscripts).
• MW(P4O10) 4(AWP) 10(AWO) 4(30.97)
  10(16.00) 283.9 g mol1
• O is 100 ? 160.0/283.9 56.36

6
Formula Weight Analysis

• Atomic Absorption Spectrometry (AA)
• Intensity of atomic glows in controlled flame
  gives proportion of atom in molecule.
• Organic combustion analysis for CxHyOz
• Burn in excess oxygen, O2
• Trap and weigh resultant H2O and CO2
• Convert weights to moles H and C, resp.
• Get mass O by difference with original mass
• Scale moles to find simplest integer x, y, z.

7
Molecular Formulae

• Empirical formula from elemental composition must
  be scaled by ratio of MWFW to obtain molecular
  formula.
• Standard formulae show combinations Hg6(PO4)2
  (why OK?)

• Structural formula give geometric information as
  well ClH2CCH2COOH, 3-chloropropanoic acid,
  digests to C3H5O2Cl
• Mineral formulae show cocrystalites like
  Y2(CO3)33H2O

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Chemical Equations

• How many reactants ? how many products?
• Chemical equations not only codify the perfect
  proportions but also note conditions
• CaCO3(s) 2 HCl(aq) ? Ca2(aq) 2 Cl(aq) .
  CO2(g)
  H2O(l)
• Catalysts, photons (h?), or heat (?) may stand
  above the reaction arrow (?).
• The key is molecular consumption/production.

9
Balancing Chemical Reactions

• First know all the reactants and products!
• Balance first atoms appearing in only one
  molecule on each side. Lets burn TNT
• C7H5(NO2)3 O2 ? 7CO2 H2O N2
• C7H5(NO2)3 O2 ? 7CO2 H2O 1.5N2
• C7H5(NO2)3 O2 ? 7CO2 2.5H2O 1.5N2
• C7H5(NO2)3 5.25O2 ? 7CO2 2.5H2O 1.5N2
• 4C7H5(NO2)3 21O2 ? 28CO2 10H2O 6N2

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Stoichiometric Calculations

• While TNT is solid, the O2 and the products are
  gases. 1 mol TNT is worth ? moles gas?
• 4C7H5(NO2)3 21O2 ? 28CO2 10H2O 6N2
• ? Moles gas ¼(28 10 6 21) 5.75
• Since each gas mole is 1000 the volume of a
  solid mole, TNT is destructive by rapidly
  increasing its volume by a factor of 5,750!

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More Calculations

• A megaton of TNT (measure of nuclear bomb
  destructivity) would produce what weight of CO2?
• 106 tons 2000 lbs / ton 0.4536 kg / lbs
  9.072?108 kg
• MWC7H5(NO2)3 7(12)5(1)6(16)3(14) 227 g
  0.227 kg
• 4C7H6(NO2)3 21O2 ? 28CO2 10H2O 6N2
• 9.072 ? 108 kg TNT ? 1 mol TNT/0.227 kg TNT ?
  28 mol CO2/4 mol TNT ? 0.044 kg CO2/1
  mol CO2
• 1.23 ? 109 kg CO2 1.23 megatons CO2

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Limiting Reactant Calculations

• Fe2O3(s) 2 Al(s) ? Al2O3(s) 2 Fe(l)
• What weight of molten iron is produced by 1 kg
  each of the reactants?
• Needed weights are MW(Fe2O3) and 2AW(Al) or 159.7
  g Fe2O3 and 53.96 g Al, respectively.
• Smallest ratio AvailableNeeded is limiting!
• 1000 g/159.7 g 6.26 Fe2O3 18.52 Al
• 6.26 mol Fe2O3 ? 2 mol Fe/1 mol Fe2O3 ? 55.85
  g Fe/1 mol Fe 699 g Fe