2. Zumdahl’s Chapter 3 Stoichiometry

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

4. 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. 98.892 % 12 C at 12 amu by definition. 1.108 % 13 C at 13.00338 amu average C  12.011 atomic mass units

5. Avogadro’s Mole Definition: One mole of 12 C atoms weighs exactly 0.012 kg (12 g) Thus, 1 amu  1 g / N Av Since atoms combine by numbers, N Av has the advantage of showing combinations by weights. N Av = 6.022136 7  10 23, the SI count

6. 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(P 4 O 10 ) = 4(AW P ) + 10(AW O ) = 4(30.97) + 10(16.00) = 283.9 g mol –1 % O is 100%  160.0/283.9 = 56.36%

7. Formula Weight Analysis Atomic Absorption Spectrometry (AA) Intensity of atomic glows in controlled flame gives proportion of atom in molecule. Organic combustion analysis for C x H y O z Burn in excess oxygen, O 2 Trap and weigh resultant H 2 O and CO 2 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.

8. Molecular Formulae Empirical formula from elemental composition must be scaled by ratio of MW:FW to obtain molecular formula. Standard formulae show combinations: Hg 6 (PO 4 ) 2 (why OK?) Structural formula give geometric information as well: ClH 2 CCH 2 COOH, 3- chloropropanoic acid, & digests to C 3 H 5 O 2 Cl Mineral formulae show cocrystalites like Y 2 (CO 3 ) 3 3H 2 O

9. Chemical Equations How many reactants  how many products? Chemical equations not only codify the perfect proportions but also note conditions: CaCO 3 (s) + 2 HCl(aq)  Ca 2+ (aq) + 2 Cl – (aq). + CO 2 (g) + H 2 O( l ) Catalysts, photons (h ), or heat (  ) may stand above the reaction arrow (  ). The key is molecular consumption/production.

10. First know all the reactants and products! Balance first atoms appearing in only one molecule on each side. Let’s burn TNT: C 7 H 5 (NO 2 ) 3 + O 2  7CO 2 + H 2 O + N 2 C 7 H 5 (NO 2 ) 3 + O 2  7CO 2 + H 2 O + 1.5N 2 C 7 H 5 (NO 2 ) 3 + O 2  7CO 2 + 2.5H 2 O + 1.5N 2 C 7 H 5 (NO 2 ) 3 + 5.25O 2  7CO 2 + 2.5H 2 O + 1.5N 2 4C 7 H 5 (NO 2 ) 3 + 21O 2  28CO 2 + 10H 2 O + 6N 2 Balancing Chemical Reactions

11. Stoichiometric Calculations While TNT is solid, the O 2 and the products are gases. 1 mol TNT is worth ? moles gas? 4C 7 H 5 (NO 2 ) 3 + 21O 2  28CO 2 + 10H 2 O + 6N 2  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!

12. More Calculations A “megaton” of TNT (measure of nuclear bomb destructivity) would produce what weight of CO 2 ? 10 6 tons [ 2000 lbs / ton ] [ 0.4536 kg / lbs ] = 9.072  10 8 kg MW[C 7 H 5 (NO 2 ) 3 ] = 7(12)+5(1)+6(16)+3(14) = 227 g = 0.227 kg 4C 7 H 6 (NO 2 ) 3 + 21O 2  28CO 2 + 10H 2 O + 6N 2 9.072  10 8 kg TNT  [1 mol TNT/0.227 kg TNT]  [28 mol CO 2 /4 mol TNT]  [0.044 kg CO 2 /1 mol CO 2 ] 1.23  10 9 kg CO 2 = 1.23 megatons CO 2

13. Limiting Reactant Calculations Fe 2 O 3 (s) + 2 Al(s)  Al 2 O 3 (s) + 2 Fe( l ) What weight of molten iron is produced by 1 kg each of the reactants? Needed weights are MW(Fe 2 O 3 ) and 2AW(Al) or 159.7 g Fe 2 O 3 and 53.96 g Al, respectively. Smallest ratio Available:Needed is limiting! 1000 g/159.7 g = 6.26 Fe 2 O 3 < 1000 g/53.96 g = 18.52 Al 6.26 mol Fe 2 O 3  [2 mol Fe/1 mol Fe 2 O 3 ]  [55.85 g Fe/1 mol Fe] = 699 g Fe