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Percent Composition. Can be calculated if given: masses of elements in compound OR the chemical formula. 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.

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Percent composition
Percent Composition

  • Can be calculated if given:

    masses of elements in compound

    OR

    the chemical formula


Percent composition1
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


Empirical formula
Empirical Formula

  • shows the simplest mole ratio of the elements.

  • CO is a 1:1 ratio of carbon to oxygen

  • H2O is a 2:1 ratio

  • CO2 is a 1:2 ratio

  • Empirical formulas can’t be reduced.


Molecular formula
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


Empirical
Empirical?

  • CH4O

    • yes, cannot be reduced further

  • C2H6

    • no, empirical would be CH3

  • C3H10O

    • yes

  • C6H6O2

    • no. What would empirical be?

    • C3H3O


Calculating empirical formulas
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?


Method
Method

  • Write the mass (g) of each element in the compound. So….

    25.9% N = 25.9g

    74.1% O = 74.1g


Percent composition

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


Percent composition


Percent composition

  • Write the empirical formula. number of moles by the smallest number of moles.

    N2 O5

    • For inorganic compounds, write the most positive element first.

    • For organic compounds, write C first, H second and all others alphabetically.


A special present just for you
A special present just for you…….. number of moles by the smallest number of moles.

  • Page 135, Problems #20 & 21


Molecular formula1
Molecular Formula number of moles by the smallest number of moles.

Given the empirical formula and the gram formula mass (gfm)

OR

Given the percent composition and the gram formula mass (gfm)


Example 1
Example #1 number of moles by the smallest number of moles.

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


Example 2
Example #2 number of moles by the smallest number of moles.

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


Percent composition

  •  convert to moles number of moles by the smallest number of moles.

    C = 60.0g/mol = 5

    12.0g

    H = 10.0g/mol = 10 1.0g

    O = 32.0g/mol = 2

    16.0g


Percent composition

number of moles by the smallest number of moles. Use moles as subscripts for components of compound

C5H10O2

  • Check the gmm of this compound…does it equal 102.0g/mol?

  • 5(12.0) + 10(1.0) + 2(16.0) = 102.0g/mol

  • YES!


And now
And Now….. number of moles by the smallest number of moles.

  • Oh Yeah! And there’s more…

  • Page 136, Problems #22 & 23


Percent composition