10-3: Empirical and Molecular Formulas

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10-3 Empirical and Molecular Formulas
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Percentage Composition

• The mass of each element in a compound, compared
  to the mass of the entire compound (multiplied by
  100)
• Tells you the percent of the mass made up by each
  element in the compound.

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How to calculate comp

• (grams of X)/(total grams) x 100
• Complete sample problem 7 on pg 334
• Complete practice problem 14 on pg 334

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Empirical Formula

• The smallest whole number ratio of the atoms of
  the elements in a compound
• Remember glucose?
• What is its molecular formula?
• What is its empirical formula?

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Calculating Empirical Formula

• 1) If given as percents, read them as grams
  (accurate if a 100 g sample)
• 2) Convert each gram amount to moles
• 3) Find the smallest whole number ratio by
  dividing each mole amount by the smallest number.
  Round as appropriate.
• 4) Use that ratio to write the empirical formula

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Sample Problem 8 (pg 36)
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Molecular Formula

• Gives the actual number of atoms of each element
  in a molecular compound.
• Is a whole number multiple of the empirical
  formula.

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Finding the Molecular Formula

1. You will be given the molar mass of the molecular
   formula in the question.
2. Find the empirical formula.
3. Find the molar mass of the empirical formula.
4. Divide molecular formula mass by empirical
   formula mass.
5. This is the whole number multiple that you should
   multiply the empirical formula by.

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Practice problem 18 (pg 339)

• Sample problem 9 (pg 338)