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DETERMINING CHEMICAL FORMULAS

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Title: DETERMINING CHEMICAL FORMULAS


1
DETERMINING CHEMICAL FORMULAS
2
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!

3
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.

4
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.

5
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)

6
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.

7
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.

8
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!)

9
Behold!...The Mass Spec!
10
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.

11
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.

12
Thats All I Got!...
  • I know its a lot to think about
  • So I will leave you to it.
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