Percent Composition and Empirical Formulas

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Percent Composition and Empirical Formulas
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Percent composition of compounds

• You can calculate the percentage that a certain
  type of atom makes up in a compound
• Mass mass of atom in 1 mole
• Total mass of 1 mole of compound

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Percent composition of compounds

• What is the mass of hydrogen in H2O?
• So first you have to find the mass of each atom
• Hydrogen 1.008 g x 2 2.016 g
• Oxygen 16.00 g x1 16.00 g
• Total 18.02 g
• Then you have plug into the equation
• H 2.016 g in 1 mole of H2O
• 18.02 Total grams in H2O
• H .1118 or 11.18

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Percent composition of compounds

• What is the percent of Carbon in glucose
  (C6H12O6)?
• C 12.01 x 6 72.06 g
• H 1.008 x 12 12.10 g
• O 16.00 x 6 96.00 g
• Total 180.2 g/ mole of glucose
• C 72.06 grams C
• 180.2 total grams
• C .4000 or 40.00

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Percent composition of compounds

• Mass percentages express the concentration of a
  component in a mixture or an element in a
  compound. For example, household bleach is 5.25
  NaOCl by mass, meaning that every 100 g of bleach
  contains 5.25 g of NaOCl. Mass percentage can be
  calculated as 100 times the mass of a component
  divided by the mass of the mixture containing the
  component.

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Empirical Formulas

• The simplest whole number ratio of atoms in a
  compound
• A molecular formula is the same as or a multiple
  of the empirical formula, and is based on the
  actual number of atoms of each type in the
  compound.
• For example, if the empirical formula of a
  compound is C3H8 , its molecular formula may be
  C3H8 , C6H16 , etc.

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Calculating Empirical Formulas

• You can calculate the empirical formula for a
  compound based on the masses of each of the
  elements that make it up.

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Calculating Empirical Formulas

• If you have an Aluminum and an Oxygen and you
  know that you have 4.151 g of Al and 3.692 g of
  Oxygen.
• First you figure out the moles of each
• Al 4.151/26.98 .1539 mol
• O 3.692/16.00 .2308 mol
• Then you divide each by the smaller of the 2
  moles (this converts it to a ratio)
• Al .1539/.1539 1
• O .2308/.1539 1.5
• Then you need to multiply to figure out the
  smallest whole number integer
• Al 1 x 2 2
• O 1.5 x 2 3
• So the answer is 2 Al/ 3 O or Al2O3

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Calculating Empirical Formulas

• Obtain the mass of each element present
• Determine the moles present of each element
• Divide the number of the smallest number of moles
  to convert the smallest number to 1. If you NEED
  to convert them to whole numbers

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Homework

• Read pages 171-176. Problems 27,30,32,33 on page
  188-189