Homework 5 Help

1
Homework 5 Help

• Working in moles

2
Definitions

• One mole 12C 12 g (exact)
• 6.02?1023 molecules 12C
• This number has a name--what is it?

3
Definitions

• One mole 12C 12 g (exact)
• 6.02?1023 molecules 12C
• Avagadros number
• Corollary One mole of anything
• ? its molecular weight in grams

4
Examples

• 1 mol H ? 1 g
• 1 mol O ? ???

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Examples

• 1 mol H ? 1 g
• 1 mol O ? 16 g
• 1 mol O2 ? ???

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Examples

• 1 mol H ? 1 g
• 1 mol O ? 16 g
• 1 mol O2 ? 32 g
• 1 mol H2O ? ???

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Examples

• 1 mol H ? 1 g
• 1 mol O ? 16 g
• 1 mol O2 ? 32 g
• 1 mol H2O ? 18 g
• Homework 1 mol air ? ???
• Molecular weights O2-32, N2-28, Ar-40
• (Neglect CO2 in calculating mean molecular wt.)

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Averages

• What is the mean (average) molecular weight of
  air?
• If air was 50 N2 and 50 O2, the average would
  be
• 0.5(28) 0.5(32) 30
• If air was 75 N2 and 25 O2, the average would
  be
• 0.75(28) 0.25(32) 29

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Why are moles useful?

• Answer Stoichiometry!
• Chemicals react on a molar basis
• CH2O O2 ? CO2 H2O
• 1 mol 1 mol 1 mol 1 mol
• Hence,one needs to convert mass into moles in
  order to figure out how much O2 reacts with a
  given mass of CH2O

10
Unit Conversions

• 1 Gt 109 tons
• 109 103 kg
• 109 103 103 g
• 1 Gt 1015 g
• 1 Gt(C) 1015 g C ? 1 mol
• 12 g
• 8.33?1013 mol C

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Percentage Change

• Suppose you have a reservoir of size R
• Suppose you increase the size of the reservoir by
  an amount ?R
• The percentage change is then
• change ?R ? 100
• R