Empirical Formulas
Definition
Ex:
A formula that gives the simplest whole
number ratio of the different atoms in a
compound.
H2O2 = hydrogen peroxide
HO = empirical formula for hydrogen
peroxide
C6H12O6 = Glucose
Empirical Formula? =
Empirical Formula Calculation
Empirical formulas are determined
from % composition.
Example:
The % composition by mass of a
compound is 56.6% K, 8.7% C, and
34.7% O. Find it’s empirical formula
Empirical Formulas
Steps
1. Assume 100g of the compound.
(Change % to g)
K = ______g
C = ______g
O = ______g
2. Convert g to moles.
(HINT: Use dimensional analysis)
Empirical Formulas
Show
your
DA!
K = 1.45mol
C = .72mol
O = 2.17mol
3. Divide all moles by the smallest
mole value.
K = 1.45mol/.72mol = 2.0
C = .72mol/.72mol = 1.0
O = 2.17mol/.72mol = 3.0
Solve the following:
1. A compound is 80% C and 20% H. What is the
empirical formula? (Follow the steps).
2. A compound was analyzed and found to
contain 13.5g Ca, 10.8g O and .675g H. What
is the empirical formula for the compound?
Solve the following:
3. A compound is 20.8% Magnesium,
42.7% Arsenic, and 36.5% O. What is
the empirical formula?
Write the name of the compound!
Molecular Formulas
Definition
Show the types and numbers of atoms in a
compound. (H2O2)
Same as the empirical formula or a whole
# multiple of it.
Example: C6H12O6 = molecular formula of glucose.
CH2O= empirical formula of glucose (or any
sugar)
H2O = empirical and molecular formula of
water
Molecular Formulas
Needed:
Need (or need to be able to calculate):
Empirical Formula (you may need to
find this )
Formula Mass or Molar Mass of
Molecular Compound (must be
given)
Molecular Formulas
Solve
The empirical formula of a compound
is P2O5 and it has a formula mass of
283.889g. What is the molecular
formula of the compound?
Molecular Formulas- Steps
1. Molar mass of the molecular compound.

283.889g (from problem)
2. Empirical formula of the compound.

P2O5 (from problem)
3. Molar mass of the empirical formula.

2(Mass of P) + 5(Mass of O) = 141.943g
4. Molar mass of molecular formula (must be the one
given)/ molar mass of empirical formula (always the
same or smaller #)

283.889g / 141.943g =2.0000
5. Multiply subscripts in empirical formula by answer
above

2(P2O5 ) = P4O10
Remember!
By definition, the molecular formula is the
same as or a whole number multiple of
the empirical formula…
If your number did not come out perfectCHECK YOUR MATH!
Solve the following:
4. A compound has 7.6g Nitrogen, 17.4g
Oxygen, and a molar mass of 138g.
What is the empirical formula?
What is the molecular formula?
Write the name of the compound!