Percent Composition, Empirical
Formula and Molecular Formula
10.4 summary
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
Percent Composition of a Compound
from experimental data
• ex: a lab sample contains 24.02 g carbon and
64.00 g oxygen.
element mass
%composition =
x100
compound mass
• 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%
Percent Composition of a Compound
from a chemical formula
• ex: find the percent composition of CO2.
element mass in1molecompound
%composition =
x100
molar mass ofcompound
• 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%
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
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
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.
Calculating Molecular Formula
1. Find molar mass of empirical formula.
2. Divide experimentally determined molar
mass (given), by molar mass of empirical
formula.
givenMM
n=
empiricalMM
1. Has to equal a whole number “n”
2. Multiply each subscript by n.
molecular formula = XnxAYnxBZnxC