1. Zumdahl’s Chapter 3
Stoichiometry

2. Chapter Contents Aston’s Atomic Mass The Mole Molar Mass % Composition
Molecular Formulae
Chemical Equations
Balancing Chemical Equations
Stoichiometric Calculations
Limiting Reactant Calculations

3. Aston’s 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.
% 12C at 12 amu by definition.
1.108 % 13C at amu
averageC  atomic mass units

4. Avogadro’s Mole
Definition: One mole of 12C atoms weighs exactly kg (12 g)
Thus, 1 amu  1 g / NAv
Since atoms combine by numbers, NAv has the advantage of showing combinations by weights.
NAv = 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) = g mol–1
% 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 MW:FW 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 co•crystalites like Y2(CO3)3•3H2O

8. 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. Let’s burn TNT:
C7H5(NO2)3 + O2  7CO2 + H2O + N2
C7H5(NO2)3 + O2  7CO2 + H2O + 1.5N2
C7H5(NO2)3 + O2  7CO H2O + 1.5N2
C7H5(NO2) O2  7CO H2O + 1.5N2
4C7H5(NO2)3 + 21O2  28CO2 + 10H2O + 6N2

10. 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( – 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!

11. More Calculations
A “megaton” of TNT (measure of nuclear bomb destructivity) would produce what weight of CO2?
106 tons [ 2000 lbs / ton ] [ kg / lbs ] = 9.072108 kg
MW[C7H5(NO2)3] = 7(12)+5(1)+6(16)+3(14) = 227 g = 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

12. 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 g Fe2O3 and g Al, respectively.
Smallest ratio Available:Needed is limiting!
1000 g/159.7 g = 6.26 Fe2O3 < 1000 g/53.96 g = Al
6.26 mol Fe2O3  [2 mol Fe/1 mol Fe2O3]  [55.85 g Fe/1 mol Fe] = 699 g Fe