Percent Composition, Molecular and Empirical Formulas

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Percent Composition, Molecular and Empirical
Formulas

• whew, say that 5 times fast

2
Percent Composition

• Have you ever seen CSI?
• You know when the lab people take a chemical and
  put it into this magical machine and it give a
  print out with the name of the compound yeah,
  that machine doesnt exist

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Percent Composition

• Scientist use percent composition as a way to
  identify some compounds
• the percentage by mass of each element in a
  compound
• For example, in 1962 scientist made a new
  compound that they found was 63.7 Xe and 36.7
  F
• This let them figure out that the formula was XeF4

4
Percent Composition

• Toluene C6H5CH3
• What is the percent composition of carbon in this
  molecule?
• carbon
• (mass of carbon)/(mass of molecule)
• Of hydrogen?

5
Empirical Formula

• Data from the percent composition lets us figure
  out the empirical formula for a compound
• Shows the simplest ratio of a compound.
• Example
• Molecular formula H2O2 C6H12O6
• Empirical formula HO CH2O

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Empirical Formula

• To find the empirical formula
• calculate the number of moles of each element in
  a compound
• Find the ratio of the elements to each other
• If you are given percentages to start (and not
  masses), assume a 100g sample

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Empirical Formula

• A compound is 38.77g of Cl and 61.23g O. What is
  its empirical formula
• A compound is made up of 18.0 C, 2.26 H, and
  79.7 Cl. What is its empirical formula?

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Molecular Formula

• If you know the molecular weight of a compounds
  (or the molar mass) and its empirical formula,
  you can then determine its Molecular formula
• a chemical formula that shows the number and
  kinds of atoms in a molecule
• It is always a whole-number ratio
• n(empirical formula) molecular formula

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Molecular Formula

• A compound has an experimental molar mass of
  78g/mol. Its empirical formula is CH. What is its
  molecular formula?