Empirical and Molecular Formulas

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Empirical and Molecular Formulas
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Empirical Formula

• Empirical vs. Molecular Formulas
• Molecular Formula a formula that specifies the
  actual number of atoms of each element in one
  molecule of a compound.
• Empirical Formula a formula with the smallest
  whole-number mole ratio of the elements that make
  up a compound.

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Empirical Formula

• Empirical Formula
• May or may not be the same as the molecular
  formula
• Molecular formula is always a simple multiple of
  the empirical formula
• ex. H2O2
• Empirical formula is HO
• Molecular formula is TWO times the empirical
  formula

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How to calculate an empirical formula

• How to calculate
• STEP 1 You will be given either masses or
  percent composition.
• STEP 2 If you are given composition, turn it
  into grams by assuming a 100.0 g sample. NOTE If
  you are given mass, you do not need to do this
  step.
• STEP 3 Convert the masses to the number of moles
  of each element.

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• STEP 4 Figure out the proportion of moles of
  each element in the compound by dividing each by
  the smallest number of moles.
• STEP 5 If step 4 resulted in whole numbers, you
  are done! However, if there were decimals, you
  will need to multiply by small, whole numbers
  until you have whole numbers.

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An example

• STEP 1
• Compound is 40.05 S and 59.95 O
• STEP 2
• I assume 100 g of the compound, so it is
• 40.05 g S and 59.95 g O
• STEP 3
• 40.05 g S(1 mol S/32.07 g S) 1.249 mol S
• 59.95 g O(1 mol O/16.00 g O) 3.747 mol O

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Continued

• STEP 4
• 1.249 mol S 3.747 mol O
• Divide each by 1.249 (smallest number in ratio)
• 1 mol S 3 mol O
• STEP 5
• SO3
• You are done! The compound is sulfur trioxide.

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A way to remember those steps

• A Poem by Joel Thompson
• Percent to mass
• Mass to mole
• Divide by small
• Multiply til whole

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Molecular Formula

• Molecular Formula this tells us how many atoms
  of each type there really are in the compound.
• Can two substances have the same empirical
  formula but be different?
• YES! Benzene vs. acetylene C6H6 vs. C2H2
• What is their empirical formula? How is this
  different from ionic compounds?

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Calculating Molecular Formula

• STEP 1
• You will be given the molar mass of the compound
  and the empirical formula.
• STEP 2
• Calculate the empirical mass (mass of the
  empirical formula).
• STEP 3
• Divide the given molar mass by the empirical
  mass. You should get a small whole number.
• STEP 4
• Multiply the subscripts of the empirical formula
  with the number obtained.

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Molecular Formula Example

• STEP 1
• The empirical formula is CH2O and the molar mass
  is 180.18 g.
• STEP 2
• The empirical mass is 12.01g 21.01g 16.00g
  30.03 g
• STEP 3
• 180.18 g/ 30.03 g 6
• STEP 4
• CH2O becomes C6H12O6

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• Closure
• Acetylene is a gas that is used as a fuel for
  welding. Benzene is a liquid solvent.
• How are they similar?
• How are they different?
• Why are they different?
• Why is one a gas at room temperature and one a
  liquid?