DETERMINING CHEMICAL FORMULAS

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DETERMINING CHEMICAL FORMULAS
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What if you discovered the cure for cancer?

• Any new chemical that is discovered that is a
  potential health care treatment must have its
  molecular formula submitted to the Ministry of
  Health.
• How do you determine the molecular formula of
  some new compound that neither your nor anyone
  may have never seen before?
• Its actually not that hardall you need is a
  device called a mass spectrometer and some
  chemistry fundamentals some of which are
  hundreds of years old!

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The Mass Spectrometer

• A mass spectrometer is a lab instrument that
  measures the molar mass of a compound.
• The machine can also give you a reading of each
  element in the compound based on mass and
  percentage.
• Look at this reading from the Cassini Spacecraft
  as it flew by one of Saturns moons Enceladus.

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Law of Constant Composition

• A compound contains elements in certain fixed
  proportions (ratios) and in no other
  combinations.
• This law is important in formula determination
  because it basically says that each compound has
  only one formulaif you find something that
  matches a known formula then that is what you
  have found no arguments.
• Water will always be H2OFind that and you have
  found water.

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Empirical vs. Molecular

• Before you can determine the molecular formula of
  a compound you must first determine the empirical
  formula.
• An empirical formula is the lowest whole number
  ratio of elements in a compound.
• Think of empirical as a formula in lowest terms.
• The molecular formula of a compound is the actual
  number of atoms in one molecule of a compound.
• Think of molecular formulas as seeing the
  compound out on the street.
• Example Glucose C6H12O6 (molecular formula)
• CH2O (empirical formula)

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Determining Empirical Formulas

• In order to determine the empirical formula for a
  compound you need the percent composition for it
• The mass spectrometer will give you thisor the
  question will tell you!
• Lets try an example of a problem in which we
  must determine the percent composition.

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Sample Problem 1 The substance that gives sour
milk that lovely taste is lactic acid. It
consists of 40.00 Carbon and 6.71 Hydrogen with
the remainder being oxygen. Determine its
empirical formulaOr Else!!!

• Steps to Solve
• Assume 100g
• ( ? g)
• g ? mol
• ( by g/mol)
• Divide by lowest mol

• C 40.00g 12.01g/mol 3.33mol
• C 3.33mol 3.33mol 1
• H 6.71g 1.01g/mol 6.64mol
• H 6.64mol 3.33mol 2
• O 53.29g 16.00g/mol 3.33mol
• O 3.33mol 3.33mol 1
• Therefore the empirical formula for lactic acid
  is CH2O.

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Empirical Is Not Enough!

• If you look at this slideshow carefully you
  will have noticed both glucose (C6H12O6) and
  lactic acid (C3H6O3) have the same empirical
  formula but they are very different molecules
  it terms of properties and function within the
  human body!
• Many molecules can have the same empirical
  formulas so it is absolutely necessary to be able
  to determine the molecular formula for any
  compound.
• In order to determine the molecular formula we
  need two pieces of information
• Empirical Formula (or a way to get itlike
  percent compositiona la the mass spectrometer)
• Molar mass of the compound (The almighty mass
  spectrometer will give us this too!)

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Behold!...The Mass Spec!
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Sample Problem 2 Determine the molecular formula
of a compound containing 85.7 carbon and 14.3
hydrogen by mass. The molar mass of the compound
is 84g/mol. SolveOr Else!!!

• We do not have the empirical formula but we do
  have a way to get it. (by using the Percent
  Composition)!
• C 85.7g 12.01g/mol 7.136mol 7.136mol 1
• H 14.3g 1.01g/mol 14.16mol 7.136mol 2
• Therefore the empirical formula is CH2.
• Molar Mass 84g/mol 6
• Empirical Formula Mass 14.03g/mol
• This means that the molecular formula is 6 times
  larger than the empirical formula so we need to
  multiply the empirical formula by 6 to get the
  molecular formula.
• 6 x CH2 C6H12
• The Molecular formula is C6H12.

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Sample Problem 3 Determine the molecular formula
of a compound with the empirical formula P2O3 and
a molar mass of 220.0g/mol.

• We are given the empirical formula outright so
  you go directly to the second step of the solving
  process.
• Molar Mass 220.0g/mol 2
• Empirical Formula Mass 109.94g/mol
• This means that the molecular formula is 2 times
  larger than the empirical formula so we need to
  multiply the empirical formula by 2 to get the
  molecular formula.
• 2 x P2O3 P4O6
• The molecular formula is P4O6.

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Thats All I Got!...

• I know its a lot to think about
• So I will leave you to it.