Avogadro

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Avogadros Number Notes
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Avogadros Number Notes

• The word dozen represents the number 12. This
  is independent of the actual items (can be
  doughnuts, marbles, atoms, etc)
• In the same way, chemists use another word called
  a mole to represent a different number.
• One mole (it is often abbreviated to mol.) 6.02
  x 1023or602,000,000,000,000,000,000,000

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Avogadros Number Notes

• This number is important enough to get a name
  (like dozen) because it can be used to calculate
  how many atoms there are in an amount of
  substance.
• For any pure element, one mole of atoms will
  weigh exactly the number in grams corresponding
  to the atomic weight of the element (found
  underneath the element symbol on the periodic
  table).
• For instance, 1 mol. of N atoms will weigh 14.007
  gIt can also be said that 6.94 g of Lithium will
  contain one mole of Li atoms (6.02 x 1023).

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Conversions

• In order to convert one unit to another
• Given info Units to convert into Units to convert
  into etc
• Units to cancel Units to cancel etc
• The Given info is what you started with, and
  the final answer is everything multiplied and
  divided together (all units cancel out except for
  the final units that you want to finish with)

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Conversionsan example
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Avogadros Number Notes

• How many moles are there in 5 grams of Carbon?
• 5 g C x (1 mol. C atoms/12.01 g. C) 0.4 moles
  C atoms
• How many atoms are there in 5 grams of C?
• 0.4 mol C atoms x 6.02 x 1023 2.4 x 1023 C
  atomsyou wont have to do this calculation
  much
• If you have 2.2 moles of Al, how many grams?
• 2.2 mol Al x (26.98 g Al/1 mol Al atoms) 59 g
  Al
• See p. 324, 328, 329

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Avogadros Number Notes

• Calculating with moles is mostly helpful because
  you will be able to predict the exact ratios of
  elements in a substance even though the atomic
  weights of the different elements may be
  different.
• For instance, in a mole of water molecules, what
  is the ratio of actual weights of hydrogen and
  oxygen (not the ratio of of atoms)?
• 1 mole of water molecules will have 1 mole of
  oxygen atoms, and two moles of hydrogen atoms.

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Avogadros Number Notes

• To calculate the mass of 1 mole of oxygen atoms
• 1 mol O x (15.99 g O/1 mol O atoms) 15.99g O
• To calculate the mass of 2 moles of hydrogen
  atoms
• 2 mol H x (1.007 g H/1 mol H atoms) 2.01 g H
• Total mass of 1 mole of water molecules 15.99
  g 2.01 g 18.0 grams
• See p. 335-337

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Avogadros Number Notes

• Another calculation you can do is determine the
  composition (by weight) of one element in a
  compound if you know the name or the formula.
• See p. 343.

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Avogadros Number Notes

• You can also infer the actual ratios of atoms
  within compounds by comparing the weights of the
  substances used to create it (finding the
  empirical formula). A multiple of the empirical
  formula in a covalent compound is called a
  molecular formula.
• See p. 345-350.

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Hydrates

• In ionic compoundsempirical formula tells the
  ratio of atoms, and the ions form lattice
  structures of alternating ions.

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Hydrates

• Sometimes there are gaps within the ions in the
  lattice structure, leaving room for water
  molecules to join. The ratio of water molecules
  to the other ions depends on the size of the
  ions, the charge of the ions, and the type of
  lattice
• Such ionic compounds that can bind to water
  molecules are called hydrates.

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Hydrates

• We can find the of water in a hydrate in the
  same way as when we found the of a specific
  element in the composition
• See p. 353.