Percent Composition, Empirical Formula and Molecular Formula

1
Percent Composition, Empirical Formula and
Molecular Formula

• 10.4 summary

2
Percent Composition

• 1 pizza
• 2 slices pineapple
• 6 slices pepperoni
• 4 slices cheese
• What of the pizza slices are
• pineapple?
• pepperoni?
• just cheese?

(2/12) x 100 17 pineapple, (6/12) x 100
50 pepperoni, (4/12) x 100 33 cheese
3
Percent Composition of a Compound from
experimental data

• ex a lab sample contains 24.02 g carbon and
  64.00 g oxygen.
• compound mass 24.02 g 64.00 g 88.02 g
• C (24.02 g / 88.02 g) x 100 27.29 C
• O (64.00 g / 88.02 g) x 100 72.71 O
• good because 27.29 72.71 100

4
Percent Composition of a Compound from a chemical
formula

• ex find the percent composition of CO2.
• MM of C 12.01 g/mole
• MM of O 16.01 g/mole
• molar mass of compound 12.01 2(16.00) 44.01
  g/mole
• C (12.01 g / 44.01 g) x 100 27.29
• O (2X16.00 g / 44.01 g) x 100 72.71
• good because 27.29 72.71 100

5
Empirical Formula

• Empirical means determined experimentally.
• If an unknown sample is analyzed and the amount
  of each element is determined, an elementary
  formula can be created.

• smallest, whole-number ratio of moles of each
  element in the compound.
• hydrogen peroxide HO

6
Calculating Empirical Formula

1. If given , assume 100g sample.
2. Convert mass of each element to moles.
3. Divide each of these numbers by the smallest
   number to force one of them to 1.
4. If necessary, multiply by the smallest number
   possible to make each a whole number.
5. These whole numbers are the subscripts in the
   empirical formula called mole ratio.

empirical formula XAYBZC
7
Molecular Formula

• Actual number of atoms of each element in a
  molecule or formula unit.
• hydrogen peroxide H2O2
• experimental molar mass needs to be provided.

8
Calculating Molecular Formula

• Find molar mass of empirical formula.
• Divide experimentally determined molar mass
  (given), by molar mass of empirical formula.
• Has to equal a whole number n
• Multiply each subscript by n.

molecular formula XnxAYnxBZnxC