Percent Composition

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Percent Composition

• Can be calculated if given
• masses of elements in compound
• OR
• the chemical formula

2
Percent Composition

• Can be used to
• ? calculate the mass of elements in a compound
• ? determine the empirical formula of a compound
• ? determine the molecular formula of a compound

3
Empirical Formula

• shows the simplest mole ratio of the elements.
• CO is a 11 ratio of carbon to oxygen
• H2O is a 21 ratio
• CO2 is a 12 ratio
• Empirical formulas cant be reduced.

4
Molecular Formula

• shows the actual number of atoms in a molecule.
• The molecular formula for hydrogen peroxide is
  H2O2. Its empirical formula would be HO.
• Often the molecular formula is the same as the
  empirical formula H2O, CO2

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Empirical?

• CH4O
• yes, cannot be reduced further
• C2H6
• no, empirical would be CH3
• C3H10O
• yes
• C6H6O2
• no. What would empirical be?
• C3H3O

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Calculating Empirical Formulas

• A chemist with an unknown compound can easily
  figure out its percent composition, but it is
  much more meaningful to know its formula.
• EXAMPLE What is the empirical formula for a
  compound that is 25.9 nitrogen and 74.1 oxygen?

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Method

• Write the mass (g) of each element in the
  compound. So.
• 25.9 N 25.9g
• 74.1 O 74.1g

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• 2. Convert the mass of each element to moles.
• N 25.9g 1.85 mol
• 14.0g/mol
• O 74.1g 4.63 mol
• 16.0g/mol

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• Calculate the simplest whole number ratio by
  dividing the number of moles by the smallest
  number of moles.
• 1.85 4.63 1 2.5
• 1.85 1.85
• (If the result is not within 0.1 of a whole
  number, multiply all numbers by a whole number)
• 2 ( 1 2.5) 2 5

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• Write the empirical formula.
• N2 O5
• For inorganic compounds, write the most positive
  element first.
• For organic compounds, write C first, H second
  and all others alphabetically.

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A special present just for you..

• Page 135, Problems 20 21

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Molecular Formula

• Given the empirical formula and the gram formula
  mass (gfm)
• OR
• Given the percent composition and the gram
  formula mass (gfm)

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Example 1

• Calculate the molecular formula for NaO having a
  gfm of 78g.
• ? Determine the efm (empirical formula mass).
• NaO 23.0g 16.0g 39.0
• Divide the efm into the gfm.
• 78.0 2
• 39.0
• This is the conversion factor used to determine
  the molecular formula.
• Na2O2

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Example 2

• Find the molecular formula for a compound having
  a composition of 58.8 C, 9.8 H and 31.4 O and
  a gmm of 102g/mol.
• Determine the mass of each component.
• C 102g/mol x 58.8 60.0g/mol
• H 102g/mol x 9.8 10.0g/mol
• O 102g/mol x 31.4 32.0g/mol

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• ? convert to moles
• C 60.0g/mol 5
• 12.0g
• H 10.0g/mol 10 1.0g
• O 32.0g/mol 2
• 16.0g

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• ? Use moles as subscripts for components of
  compound
• C5H10O2
• Check the gmm of this compounddoes it equal
  102.0g/mol?
• 5(12.0) 10(1.0) 2(16.0) 102.0g/mol
• YES!

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And Now..

• Oh Yeah! And theres more
• Page 136, Problems 22 23

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• Now Try page 139, 41 ?44