The Mole Unit VII Ch' 11

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The Mole Unit VIICh. 11
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The Mole

• Chemical Symbols and Formulas - Represent
  elements and compounds
• Elemental symbol
• one atom of an element
• Formula of a compound
• one molecule or one formula unit

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The Mole - Groups of Items

• We often represent groups of items using such
  terms as
• Dozen (12)
• Gross (144)
• Acre (43,560 ft2 4840 yd2 160 rod2)
• Yard (3 ft)
• Yard of concrete (cubic) 3x3x3 27 ft3

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The Mole

• A term chemists use to refer to a specific number
  of atoms, ions, molecules, or formula units
• The SI base unit representing the chemical
  quantity of a substance
• 1 mole 6.02 x 1023 items
• 1 mole of particles has a mass in grams equal to
  the mass of one particle in amu.
• Thus, if a single atom has a mass of 14 amu, then
  a mole of the same substance has a mass of 14 g.

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The Mole

• amu/atom grams/mole
• 1 mole atoms 6.02 x 1023 atoms
• 1 mole molecules 6.02 x 1023 molecules
• 1 mole ions 6.02 x 1023 ions
• 1 mole formula units 6.02 x 1023 formula units

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Molar Mass (MM)Also known as Molecular Weight,
MW

• The mass of one mole of atoms, ions, molecules,
  (anything)
• Convert grams to atoms and grams to moles and
  vice versa
• Convert grams to moles, and then convert moles to
  molecules (see the worksheet on this procedure
  under the Resource section)

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Conversions

• /MW /6.02 x 1023
• Grams moles particles
• x MW x 6.02 x 1023

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Problem 1

• You have 6.02 x 1023 atoms of Ca (calcium). How
  many moles do you have?

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Answer 1

• 6.02 x 1023 x 1 mole/6.02 x 1023 atoms 1.0 mole

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Problem 2

• How many moles do you have in 11.5 g ethanol
  (C2H5OH)?
• 1 mole ethanol has a mass of 46 g

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Answer 2

• (2x12) (6x1) (1x16) 46 g
• 11.5 g C2H5OH x 1 mole C2H5OH /46 g C2H5OH
• 0.250 g

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Problem 3

• You have 1.2 x 1025 molecules NH3.
• How many moles do you have and what is its mass?

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Answer 3

• Process convert molecules to moles to grams
• 1.2 x 1025 molecules NH3 x
• 1 mole NH3/6.02 x 1023 x
• 17 g/mole 338 g NH3

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Additional Problems

• Convert 0.638 mole Ba(CN)2 to grams
• Convert 50.4 g CaBr2 to moles
• Convert 1.26 moles NbI5 to grams
• Convert 86.2 g C2H4 to moles

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Calculation of Empirical Formulas

• Atoms are present in a molecule in simple
  whole-number ratios. Therefore, the moles of
  atoms for each element are also present in
  whole-number ratios.
• If we know the number of grams of all components,
  we can calculate the ratio of these components
  (i.e., empirical formula).
• Example Total sample weight 2.50 g
• Ca 0.900 g (MW 40.1)
• Cl 1.60 g (MW 35.5)

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Empirical Formula

• Step 1
• moles Ca g Ca / MW Ca 0.900 g / 40.1 g/mole
  0.0224 moles
• moles Cl g Cl / MW Cl 1.60 g / 35.5 g/mole
  0.0451moles

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Empirical Formula

• Step 2
• Find the smallest ratio by dividing each result
  above by the smaller number of moles (0.0224 in
  this example)
• Ca 0.0224 mol / 0.0224 mol 1.00
• Cl 0.0451 mol / 0.0224 mol 2.01
• Therefore for each mole of Ca there are 2 moles
  of Cl
• CaCl2

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Calculate of Empirical Formulas from Percent
Composition

• If we had 100.0 g of the sample, then the
  composition would equal the number of grams which
  we would have.
• e.g.,
• 40.0 C, 6.71 H, 53.3 O
• ? 40.0g C, 6.71g H, 53.3g O

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• Then, solve the problem as before
• C 40.0 g / 12 g/mole 3.33 moles C
• H 6.71 g / 1.01 g/mole 6.64 moles H
• O 53.3 g / 16 g/mole 3.33 moles O
• Divide each result by the smallest number of
  moles (3.33)
• C 3.33 / 3.33 1.00 gtgt 1
• H 6.64 / 3.33 1.99 gtgt 2
• O 3.33 / 3.33 1.00 gtgt 1
• \ Empirical formula CH2O

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Calculation of Molecular Formulas

• After we have determined the empirical formula,
  we can calculate the molecular formula if we know
  the molecular mass (MM, MW)
• Empirical formula CH2O emp. mass 30.0g/mole
• If we know that the molar mass (MM) is 180, then
• molar mass / emp. mass 180 / 30 6
• Then, multiply the empirical formula by the ratio
• 6 x CH2O C6H12O6

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Hydrates

• Compounds that crystallize from a water solution
  with water molecules adhering to the ions or
  molecules, and becoming part of the crystal.
• The ratio of water to the compound is specific.
• Formulas are written as such
• CuSO4 5 H2O
• Copper (II) sulfate pentahydrate

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Formula of a Hydrate

• BaI2 hydrate
• original mass 10.407 g
• heated to drive off water 9.520 g
• mass of water driven off 0.887 g
• mass of BaI2 (anhydrous) 9.520 g

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Formula of a Hydrate (contd)

• Calculate molar ratios
• BaI2 9.520 g / 391 g/mole 0.0243 moles 1
• H2O 0.887 g / 18 g/mole 0.0493 moles 2
• \ Formula for hydrate BaI2 2 H2O