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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

slide8
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

slide9
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

slide10
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.
a special present just for you
A special present just for you……..
  • Page 135, Problems #20 & 21
molecular formula1
Molecular Formula

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

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

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

slide15
 convert to moles

C = 60.0g/mol = 5

12.0g

H = 10.0g/mol = 10 1.0g

O = 32.0g/mol = 2

16.0g

slide16
 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…..
  • Oh Yeah! And there’s more…
  • Page 136, Problems #22 & 23
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