Empirical and Molecular Formulas

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Empirical and Molecular Formulas

• Honors Chemistry
• Mrs. Coyle

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Empirical Formula

• A chemical formula that represents the simplest
  whole number ratio of atoms in a compound.

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Problem 1

• Determine the empirical formula of C4H10 .
• Answer C2H5

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Molecular Formula

• The exact formula of a molecule.
• (Empirical Formula)n
• n Molar Mass
• Empirical Formula Mass

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Problem 2

• Determine the empirical and molecular formulas of
  a compound that has a percent composition by mass
  of
• 43.64 phosphorous and 56.36 oxygen and a
  formula mass of 283.88 g.

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Problem 2 Solution

• Assume 100g of the compound.
• mol P 43.64 g P x 1 mol P1.409 mol P
• 30.97g P
• mol O 56.36g O x 1 mol O 3.523 mol O
• 16.00 g O

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Problem 2 Solution Contd

• Find the mole ratio of the larger to the smaller
  number of moles.
• 3.523 mol O 2.5 mol O
• 1.409 mol P 1 mol P
• Simplest whole number ratio 5 mol O
• 
  2 mol P
• So, empirical formula P2O5

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Problem 2 Solution Contd

• Molecular formula calculation
• nMolar Mass______ 283.88 g 2
• Empirical Formula Mass 141.94 g
• So, Molecular Formula (P2O5)2
• P4O10