Percent Composition, Empirical and Molecular Formulas

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Percent Composition, Empirical and Molecular
Formulas
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Percent Composition

• Percentage by mass of each element in a compound
• Example Determine the of hydrogen and oxygen
  in water
• 1. Start with formula H2O
• 2. Calculate formula mass 18g/mol
• 3. Determine of total mass for each element
• H 2/18 .11 100 11 hydrogen
• O 16/18 .89 100 89 oxygen

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Practice Problem 1

• Determine the composition of Sulfur in H2SO4

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Practice Problem 2

• How much Na can be recovered from 45g of NaCl?

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Empirical vs. Molecular Formulas

• Empirical Formula shows the ratio of elements in
  a compound
• Molecular Formula shows the actual number of
  atoms in a compound
• Example glucose
• empirical CH2O
• molecular C6H12O6

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Calculating Empirical Formulas

• Assume you have 100g of a sample
• Use composition and convert to grams
• Divide number of grams of element by its molar
  mass to determine of moles
• Divide all of the mole numbers by the smallest
  mole number to determine ratios
• Write empirical formula

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Example 1

• Determine the empirical formula for a compound
  that is 80 carbon and 20 hydrogen.

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Example 2

• Determine the empirical formula for a compound
  that is 38.7 Mg and 61.3 F.

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Molecular Formulas

• To find the molecular formula, you must know the
  molar mass
• Molecular Formula (empirical formula)n
• n _____molar mass_______
• empirical formula mass

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Finding Molecular Formulas

• Example 1 A white powder was analyzed and found
  to have an empirical formula of P2O5. The
  compound has a molar mass of 283.88g. What is
  the empirical formula?
• Empirical Formula Mass P2O5 142g
• Divide actual molar mass by empirical mass
  238.88g/142g 2
• (P2O5)2 ? P4O10

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Finding Molecular Formulas

• Vitamin C has been analyzed and found to contain
  40.9 carbon, 4.58 hydrogen, and 54.5 oxygen.
  The molar mass of vitamin C is 176g. What is the
  molecular formula? (HINT find empirical formula
  first!)