The Mole and Avogadros Number

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The Mole and Avogadros Number

• 6.022 x 1023

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• Molecules and atoms are extremely small objects -
  both in size and mass. Consequently, working with
  them in the laboratory requires a large
  collection of them.
• How large does this collection need to be?

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• A standard needs to be introduced. This standard
  is the "mole". The mole is based upon the
  carbon-12 isotope.
• Carbon 12 is the most frequently occurring
  isotope of the element Carbon (98).

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Avogadro's Number!

• We ask the following question How many carbon-12
  atoms are needed to have a mass of exactly 12 g?
  That number is NA - Avogadro's number.
• Careful measurements yield a value for NA
  6.0221367x1023.
• 1 mole 6.022 x 1023 particles of any substance

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Say What???

• For example, if we stack 1 mole of pennies on top
  of one another, how tall would the stack be?
• The answer is it would be so tall that the stack
  of pennies could reach the sun and back almost
  500 million times!

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• The mole concept is no more complicated than the
  more familiar concept of a dozen
• 1 dozen 12 objects.
• 1 dozen elephants 12
  elephants
• 1 dozen feathers 12 feathers
• Remember???? Dimensional Analysis!
• 1 mole 6.022x 1023 atoms of any element!

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More Examples..

• 1 mole of gumballs is 6.022 x 1023 gumballs
• 1 mole of popcorn is 6.022 x 1023 kernels of
  popcorn
• Who weighs more??? Obviously the gumballs, but
  the quantity is the SAME for both!
• This much popcorn would cover the entire US 310
  miles high! Whew, thats a lot of poppin!

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Moles and Grams

• The mass in grams of 1 mol of a substance is its
  atomic mass.
• What is 1 mole of oxygen???
• 15.9994 grams (its atomic mass)
• What is 1 mole of calcium???
• 40.08 grams (its atomic mass)

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• 1 mole of Carbon weighs 12 grams.
• In other words6.022 x 1023 atoms of Carbon
  weighs 12 grams.
• 1 mole of Chlorine weighs 35.45 grams.
• In other words 6.022 x 1023 atoms of Chlorine
  weighs 35.45 grams.

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Atomic Mass Unit (amu)

• This is a unit that describes the mass of a
  single atom!
• One single atom of Carbon has a mass of 12 atomic
  mass units. This is very small.
• One mole of Carbon (6.022 x 1023 atoms of Carbon)
  weighs 12 grams. This is very large.

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Lets remember D.A.

• What is the mass of exactly 150 gumballs? 10
  gumballs 21.4 g
• If you want 50 eggs, how many dozens must you
  buy?
• If a football player is tackled 1.7 ft short of
  the end zone, how many more yards does the team
  need to get a touchdown?

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• 150 gumballs x 21.4 g 321 grams
• 1 10 gumballs
• 50 eggs x 1 dozen 4.2 dozen
• 1 12 eggs
• 1.7 ft x 1 yard 0.57 yards
• 1 3 ft

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NOTES! Copy these conversions.

• 1 mole 6.022 x 1023 atoms
• 1 mole molecular weight (in grams)

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Lets try one together!

• Determine the mass in grams of 5.50 moles if
  iron.
• Put the original number over 1.
• Decide where the unit needs to go in the next
  step to cancel out.
• Decide which conversion you will use, and plug in
  the numbers.

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The Answer

• 5.50 mol Fe x 55.85 g Fe 307 g Fe
• 1 1 mol Fe

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Determine the amount of iron present in 352 grams
of Fe.
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The Answer

• 352 grams Fe x 1 mole Fe 6.30 mols Fe
• 1 55.85 grams

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Practice Problems

• How many grams are in 2.50 mols of S?
• How many grams are in 3.20 mols of Cu?
• How many mols are present in 11 grams of Si?
• How many mols are present in 205 g of Hg?
• What is the mass in grams of 0.48 mol of Pt?

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Solutions!

• 80.2 g S
• 203 g Cu
• 0.39 mol Si
• 3.1 mol Hg
• 94 g Pt