Chapter 8: Chemical composition

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Chapter 8 Chemical composition
Chemistry 1020 Interpretive chemistry Andy
Aspaas, Instructor
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Atomic masses

• Atomic mass unit, amu a very small unit of mass
  in which masses of atoms and molecules are given
• A single carbon-12 atom has mass of 12 amu
• But, since there are different naturally
  occurring isotopes of carbon (carbon-12,
  carbon-13, carbon-14), the average mass of a
  carbon atom is larger than 12 amu
• Average atomic mass (atomic weight) of carbon is
  12.01 amu
• The weighted average of all isotopes in a natural
  sample

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The mole

• 2 objects 1 pair, 12 objects 1 dozen
• Grouping can allow for large quantities to be
  more easily counted
• Laboratory-sized samples of chemicals contain a
  very, very large number of atoms or molecules
• The mole is a unit chemists use to represent
  very large numbers of particles (atoms,
  molecules, ions)
• Could be used for anything though, just like
  pair, dozen

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What is a mole?

• 1 mole (mol) defined as number of atoms in
  exactly 12 g of carbon-12
• Avogadros number, NA 6.022 x 1023
  particles/mole
• Just another quantity like
• Pair 2 objects
• Dozen 12 objects
• Gross 144 objects
• Ream 500 objects
• Mole 6.02 x 1023 objects

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How big is Avogadros number?

• 1 mole of chemistry textbooks would cover the
  surface of the earth to a depth of 300 km
• If you won 1 mole of dollars when you were born
  and spent a billion dollars per second, 99.999
  would still be left at 90 years old
• A mole of pennies placed side by side would
  stretch more than a million light years

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Converting between particles and moles

• Avogadros number is the same no matter the type
  of particles
• Use 1 mol 6.022 x 1023 particles as a
  conversion factor
• Number of particles in 5.000 mol carbon?
• (Same as 5.00 mol Fe, H2O, or anything!)
• Number of moles in 5.21 x 1024 Al atoms?

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Converting between moles and mass

• Since 1 mol number of atoms in 12 g of
  carbon-12,
• 1 mol of natural carbon atoms has a mass of 12.01
  g
• The mass of 1 mol of any element is equal to its
  atomic weight (from the periodic table) in grams!
• 1 mol Fe 55.85 g, 1 mol Na 22.99 g, etc
• Use values like this from the periodic table as
  conversion factors

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Molar mass

• Idea extends past just atoms
• Once can calculate mass of 1 mol of any molecule
  too!
• Just add up atomic weights of all atoms in the
  molecule, and you get the molar mass of that
  compound
• Molar mass of H2O 2(1.008) 16.00 18.016
  g/mol

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Mass percent

• Sometimes useful to know composition of a
  compound in terms of the masses of elements
  involved
• Mass percent mass of 1 element in 1 mol of the
  compound divided by mass of 1 mol of the entire
  compound, times 100
• Percents are just fractions multiplied by 100
• Practice mass percentages of each element of
  ethanol, C2H5OH

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Empirical and molecular formulas

• Empirical formula formula that describes the
  simplest ratio of elements in a compound
• Molecular formula formula that describes the
  actual number of atoms of different elements in a
  single molecule
• Ex. Butane
• Molecular formula C4H10 (actual number of atoms
  in molecule)
• Empirical formula C2H5 (simplest whole-number
  ratio)

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Calculating empirical formulas

• If given masses of elements in a compound,
  convert each mass to moles
• Then divide all mole values by the smallest one
  to get the ratios of atoms in the compound
• If all the ratio values are integers, they become
  subscripts in the empirical formula
• If there are non-integer values, multiply them
  all by the smallest integer to make them all
  integers

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Empirical formula calculation practice

• Determine the empirical formula of the following
• A sample of phosphoric acid contains 0.3086 g
  hydrogen, 3.161 g phosphorus, and 6.531 g oxygen
• A sample of para-dichlorobenzene contains 5.657 g
  carbon, 0.3165 g hydrogen, and 5.566 g chlorine.

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Empirical formula from percent composition

• If youre given percent composition, assume you
  have a 100-g sample, and convert percentages
  directly to grams
• Ex. 52.5 carbon becomes 52.5 g carbon
• All should add up to 100 g since all percentages
  must add up to 100
• Then, work as before by converting to moles and
  finding ratios

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Calculation of molecular formulas

• If you know the molar mass, you can convert an
  empirical formula into a molecular formula
• Calculate empirical formula mass, and divide by
  molar mass
• (molar mass) / (empirical formula mass) n
• Multiply n by subscripts in empirical formula to
  get molecular formula
• Ex. empirical formula CH, molar mass 78