Zumdahl’s Chapter 3

Stoichiometry

Chapter Contents

Aston’s Atomic Mass

The Mole

Molar Mass

% Composition

Molecular Formulae

Chemical Equations

Balancing Chemical Equations

Stoichiometric Calculations

Limiting Reactant Calculations

Aston’s Atomic Masses(works for molecules too!)

Mass SpectrometerFast atomic ions (current) bend near magnetDeflection 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 amuaverageC 12.011 atomic mass units

Avogadro’s 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 = 6.02213671023, the SI count

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 mol–1

% O is 100% 160.0/283.9 = 56.36%

Formula Weight AnalysisAtomic 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 massScale moles to find simplest integer x, y, & z.

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

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.

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 7CO2 + 2.5H2O + 1.5N2

C7H5(NO2)3 + 5.25O2 7CO2 + 2.5H2O + 1.5N2

4C7H5(NO2)3 + 21O2 28CO2 + 10H2O + 6N2

Balancing Chemical Reactions

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!

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 ] = 9.072108 kg

MW[C7H5(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

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 Available:Needed is limiting!1000 g/159.7 g = 6.26 Fe2O3 < 1000 g/53.96 g = 18.52 Al

6.26 mol Fe2O3 [2 mol Fe/1 mol Fe2O3] [55.85 g Fe/1 mol Fe]

= 699 g Fe