Chapter 8: The Mole

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Chapter 8 The Mole
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Counting by weighing

• If 3 beans weigh 1 gram and you want 300 beans..
• Weigh out 100 grams instead of counting all those
  beans!

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We do the same thing when we want a certain
number of atoms, which are individually very small

But how much does an atom weigh?
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What is the approximate number of carbon atoms it
would take placed next to each other to make a
line that would cross this dot

• 4
• 200
• 30,000,000
• 6.02 x 1023

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Mass Spectroscopy
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By using Mass Spec and atoms of known weight,
scientists found that

• The mass of an 1H atom is
• 1.6727 x 10-27 kg
• The mass of a 12C atom is
• 1.9927 x 10-26 kg
• Because these are long, awkward numbers, in 1961
  scientists decided to define a new unit, the amu
  or dalton

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So,by definition, the weight of 12C 12 amu
Set up a ratio
Mass of 1H atom 1.6727 x 10-27 kg mass
of 1H atom Mass of 12C atom 1.9927 x 10-26
kg 12 amu
Then the mass of 1H atom 1.0072 amu
And 1 amu 1.66058 x 10-27 kg 1.66058 x 10-24
g
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What kind of balance will I use to weigh out 1
amu of something?
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Maybe I need a new unit
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1 atom of H weighs 1.007 amu Convert from amu
to gram 1.007 amu x 1.66058 x 10-24 g So 1
atom of H weighs 1.6719 x 10-24 g
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What kind of balance will I use to weigh out
1.6719 x 10-24 grams of something?
Maybe, as with coffee beans, I need to measure
MORE THAN ONE..
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I need to measure out 1 MOLE of atoms

• Recall that 1 atom of H weighs 1.6719 x 10-24 g
• Recall that 1 mole of atoms 6.022 x 1023 atoms
• 1.6719 x 10-24 g x 6.022 x 1023 atoms
• Atom of H Mole
• 1.0068 g per mole of H atoms
• Now THIS is an amount I can measure!

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• The weight, E, is equal to
• The weight of ONE atom, in AMU
• The weight of ONE MOLE of atoms, in GRAMS

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The weight of one mole of C atoms is

• 12.011 amu
• 6 amu
• 12.011 grams
• 6 grams

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A mole of Li atoms weighs 6.941 grams. How would
you determine the weight of one Li cation?

• (6.941 g/mole) x (6.022 x 1023 atoms/mole)
• (6.941 g/mole) (6.022 x 1023 atoms/mole)
• (6.941 g/mole) / (6.022 x 1023 atoms/mole)

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What about molecules? First, lets look at a DOG
and a DOZEN DOGS
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Wt. of 1 dog vs. Wt. of 12 dogs
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What about molecules?

• 1 molecule CH4 5 atoms TOTAL
• 1 carbon atom
• 4 hydrogen atoms

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What are the number and kinds of atoms in a
molecule of C11H22O11?

• 11 C, 11 H2, 11 O
• 11 C, 22 H, 11 O
• 5.5 C2, 11 H2, 5.5 O2

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What is the formula of a compound composed of 2
Na, 1 S, and 4O?

• Na2S1O4
• 2Na1S4O
• Na2SO4
• 2Na1S4O

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CH4 One molecule vs. One mole
Molecular Mass
Molar Mass
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What about ions?

• The weight of an electron is so small
  relative to the weights of the protons and
  neutrons that GAIN or LOSS of electrons doesnt
  affect the weight of a molecule or a mole of
  molecules

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Composition think back to the dog
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Composition, Molecule

• Mass of 1 mole of element x 100 of element
• Mass of 1 mole of compound
• Example CH2O

TOTAL (molar mass) 30.025
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The molecular weight of the compound NaOH is
40.00 g/mole. What is the of Na in the
compound?

• 22.99/40.00
• 40.00/22.99
• (40.00/22.99) x 100
• (22.99/40.00) x 100

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Empirical vs. Molecular Formula

• Empirical smallest whole number ratio of atoms
• Molecular actual number and kinds of atoms
• THEY MAY BE THE SAME

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The empirical formula is found from composition
g moles divide all
by round or
smallest multiply by
of
moles whole number
to find ratio
to remove
fraction

This empirical formula is CH
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Which of the following is an empirical formula?

• C2H2
• C4H10
• C6H5OH
• N2O4

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Your Turn
g moles divide all
by round or
smallest multiply by
of
moles whole number
to find ratio
to remove
fraction

This empirical formula is ????
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Molecular Formula is the actual number and kind
of atoms in a molecule

• Molar mass small whole number
• Empirical formula mass
• Multiply every subscript in the empirical formula
  by this small whole number to obtain the
  MOLECULAR FORMULA

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Molecular Formula Example

• The empirical formula of a compound is C3H6O2,
  and its molar mass is 142.0 g. What is its
  molecular formula?
• C3H6O2 (3 x 12.011) (6 x 1.007) (2 x 16.00)
  74.0 g
• 142.0/74.0 1.91 2
• 2 x C3H6O2 C6H12O4