Empirical Formulas and Molecular Formulas

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Empirical Formulas and Molecular Formulas
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Empirical Formulas

• shows the smallest whole-number ratio of the
  atoms in the compound
• CH only means it is a 11 ration between carbon
  and hydrogen
• could be C2H2 (ecetylene), or C6H6 (benzene), or
  C8H8 (styrene)

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Determining Empirical Formula of a Compound

• What is the empirical formula of a compound
  thats mass is 25.9 N and 74.1 O?
• if there were 100g of this compound then 25.9g
  would be nitrogen and 74.1g oxygen
• change to moles in order to convert the to a
  number
• 25.9g N x 1 mol N 1.85 mol N
• 14.0 g N
• 74.1g O x 1 mol O 4.63 mol O
• 16.0 g O

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• a formula is not just the ratio of atoms, it is
  also the ratio of moles
• this leaves a ratio of N1.85O4.63 which is not a
  whole number
• therefore, divide each by the lowest quantity (in
  this case, 1.85)
• now is N1O2.5
• again, still not a whole number, but if you were
  to double it, it would be N2O5
• the empirical formula is N2O5

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Example 2

• Calculate the empirical formula of a compound
  composed of 38.67 C, 16.22 H, and 45.11 N.
• Assume 100 g so
• 38.67 g C x 1mol C 3.220 mole C
  12.01 g C
• 16.22 g H x 1mol H 16.09 mole H
  1.01 g
  H
• 45.11 g N x 1mol N 3.219 mole N
  14.01 g N

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• 3.220 mole C
• 16.09 mole H
• 3.219 mole N
• C1H5N1 is the empirical formula

• C3.22H16.09N3.219

If we divide all of these by the smallest number
(3.22), it will give us whole numbers and
therefore the empirical formula
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Comparing Empirical and Molecular Formulas
 
Compound Molecular Formula Empirical Formula
Water H2O H2O
Hydrogen Peroxide H2O2 HO
Glucose C6H12O6 CH2O
Methane CH4 CH4
Ethane C2H6 CH3
Octane C8H18 C4H9  
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Molecular Formulas

• a molecular formula is the same as, or a multiple
  of, the empirical formula
• it is based on the actual number of atoms of each
  type in the compound
• the following have the same ratio of elements and
  therefore the same empirical formula CH2O,
  however, many molecules can have that ratio, each
  having a different molecular formula
• C2H4O2 is the molecular formula of ethanoic acid
• CH2O is the molecular formula of methanal
• C6H12O6 is the molecular formula of glucose

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• divide the experimental molar mass by the mass of
  one mole of the empirical formula
• this results in the multiplier to convert to the
  molecular formula
• calculate the molecular formula of a compound
  whose molar mass is found to be 60.0g and has an
  empirical formula of CH4N
• experimental molar mass --- 60.0 g
• empirical molar mass --------- 30.0 g
• 2 (CH4N) C2H8N2

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Determining the empirical formula and then
molecular formula of a compound

• A compound is known to be composed of 71.65 Cl,
  24.27 C and 4.07 H. Its molar mass is known is
  known to be 98.96 g. What is its molecular
  formula?
• need to calculate the empirical formula first
• assume 100 g so
• 71.65 g Cl x 1mol Cl 2.02 mole Cl
  
  35.5 g Cl
• 24.27 g C x 1mol C 2.02 mole C
  12.0 g C
• 4.07 g H x 1mol H 4.07 mole H
  1.0 g H

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• Cl2.02C2.02H4.07
• divide by lowest (2.02 mol )
• Cl1C1H2 is the empirical formula
• this would give an empirical formula mass of 48.5
  g
• recall the problem asked for the molecular
  formula of a compound with a molar mass of 98.96
  g which is twice that of 48.5 g
• therefore, Cl2C2H4 is the molecular formula