Empirical Formula vs. Molecular Formula

1
Empirical Formula vs. Molecular
Formula

• Empirical formula the formula for a compound
  with the smallest whole-number mole ratio of the
  elements
• Molecular formula formula for a compound with
  the actual number of atoms of each element in a
  molecule or formula unit of the substance
• What is the difference between a molecule and a
  formula unit?

2
Empirical Formula vs. Molecular
Formula, continued

• Example of the difference Benzene is composed of
  6 carbon and 6 hydrogen (C6H6).
• What is the ratio of carbon-to-hydrogen in
  benzene?
• If we reduce the compound formula to its
  smallest whole-number mole ratio of the
  elements (empirical formula), we are left with
  CH
• Acetylenes molecular formula is C2H2. What is
  its empirical formula?

3
Calculating Empirical Formula From Percent
Composition

• If given percent composition of a compound, make
  the percent of each element that elements mass
  in grams, so the whole compound has a molar mass
  of 100 grams.
• Convert the mass of each element to moles.
• If the answers (moles) are not whole numbers,
  divide all answers by the smallest number of
  moles present.
• If still not whole numbers, multiply all answers
  by the smallest factor that will give you whole
  numbers.

4
Calculating Empirical Formula From Percent
Composition, continued..

• Examples P.349 (144, 147)
• Practice Problems (p.333) --- reference Example
  Problem on p.332 for needed guidance