september 21, 2001

(exam and review-- separate notes)

see also Background notes (What you should know) published around Sept 14)

Mole and Chemical Bookkeeping

• Atoms combine with each other to form compounds
  • for each molecule
  • the atom ratio is simple integers
  • the ratio is fixed
    • H₂O, C₂H₆, Fe₂O₃
• Molecules react in similar manner
  • simple integers
  • combining ratios
  • expressed with balanced equation
    • 2 H₂O + Ca ----> Ca(OH)₂ + H₂ (gas)
  • if you add extra water, just get wet product
  • if you add extra Ca, some is left over
• There are many ways to express amount
  • by mass (weight)
  • by volume
  • by counting (number of boxes or cans)
    • useful if box/can is standard size
  • by items
    • easy to recognize (silverware, chairs, people)
    • often in multiples (ream=1000 sheets, dozen, box with 1000 bolts)
• To chemists-- the amount is logically #atoms or molecules
  • reactions-- combine in this ratio
  • simple, predictable
  • but atom is extremely tiny
• Integer ratios common beyond chemistry
  • formal dinner-- 1 person, 1 chair, 2 forks, 6 Brussel Spouts , 1/8 of a pie
  • assemble steel frame
    • girder takes 8 large bolts
    • each bolt takes 2 small washers and 1 moderate sized nut
    • easy to plan (count, dozens, hundreds, pkgs of 5)
  • what if bolt, nut and washer sold by weight?
    • certainly won't buy 5 lbs of bolts, 10 lbs of washers and 5 lbs of nuts

Back to Chemistry

• we won't count molecules
• we know mass of each atom, molecule varies by type
• we adopt new units
  • 1. simple mass units (AMU, atomic mass unit)
    • H-nearly 1.0 (1.008), O nearly 16, Cl is 35.5
    • 1= mass of proton or neutron (nearly)
    • easy to understand O=16, N=14, U=238
    • why is Cl 35.5?
      • mixture of two types Cl35 and Cl37
      • any ordinary sample is a mixture
      • atomic weight is an average
  • 2. For molecules, we add the masses of the atoms
    • using AMU
    • use the average if mixed isotopes
    • CO2 -- Molecular Weight is C=12 + 2 x O=16 = 44
    • Na2SO4 (ionic, no molecules) -- Formula Weight = 2X23 (Na) + 32 (S) + 4x16 (O)
  • 3. we adopt a definition of a mole
    • a mole is a collection of 6.023 x 1023 particles
      • (atoms, molecules, forks)
    • the number is historical "accident"
    • weigh out 1 formula weight in grams
    • this contains 1 mole of the material
  • 4. weigh a sample, in grams
    • divide by Formula Weight
    • result is the number of moles

Once we have amounts, in moles, we can discuss what happens

• how much CO₂ can you get from burning 1 g of glucose
  • glucose = C₆H₁₂O₆
  • reaction C₆H₁₂O₆ + 6 O₂ ---> 6CO₂ + 6H₂O
  • formula weight is 6x12 + 12x1 + 6x16 = 180
  • # moles = 1g/ 180 = .00555
  • # moles of CO₂ = 6 x .000555= 0.03333
  • wt of one mole of CO₂ = 44 (AMU)
  • wt of CO₂ produced = 44 x .003333 = 1.46 g

• Two forms of a question:
  • how much Fe2O3 can I make from 1 g of F and 1 g of O₂
  • how much Fe2O3 can I make from one mole of Fe and one Mole of O₂
  • #2 is much easier

1. if all Fe reacts-- I get 0.5 moles
2. if all the O reacts-- I get 2/3 mole
3. I'll run out of Fe first
4. so I get 0.5 moles of Fe₂O₃ and have some O₂ left over