Chapter 11 Notes IV
Empirical/Molecular Formula
Empirical Formula
The empirical formula gives you
the lowest, whole number ratio
of elements in the compound.
The empirical formula may or may
not be the same as the molecular
formula.
For example:
For carbon dioxide, the molecular
formula is CO2, and the empirical
formula is CO2. One carbon and
two oxygens are the lowest ratio
of atoms.
The molecular formula for
dinitrogen tetrahydride is N2H4,
but the empirical formula is NH2.
What is the empirical formula for:
C6H12O6
C6H12O2
N2H2
CH4
Obj. 10-11…Empirical
Formulas
• empirical formulas are the lowest whole # ratio of a
compound.
• i.e. the empirical formula for glucose (C6H12O6) would be…
CH2O
• to calculate from % composition…
1. ÷ given % by atomic mass (to find moles)
2. ÷ each mole in step 1 by lowest mole value (to find ratio)
3. values from step 2 = subscripts
~ subscripts MUST be a whole # (multiply if they are not!)
To Find the Empirical Formula from
% Composition:
If given the percentages, assume
there are 100.0 grams of the
compound.
Convert the grams of each element
to moles.
Divide by the smaller amount of
moles, then manipulate the ratio
so that all numbers are whole.
Obj. 10-11 cont…
• practice…
1. What is the empirical formula of a compound that
contains 36.5% sodium, 25.4% sulfur and 38.1% oxygen?
Na = 36.5%
= 1.59 moles
=
23
0.794
2
S = 25.4%
= 0.794 moles ***
=
32
0.794
O = 38.1%
= 2.38 moles
=
16
0.794
3
1
subscripts
Na2SO3
Obj. 10-11 cont…
• more practice…
2. What is the empirical formula of a compound that
contains 72.4% iron (Fe) and 27.6% oxygen?
Fe = 72.4%
= 1.293 moles ***
=
56
1.293
O = 27.6%
= 1.725 moles
=
16
1.293
1x3= 3
1.33 x 3 = 4
MUST be a whole #!!!
Fe 3O4
Two Examples:
What is the empirical formula of a
compound that is 27.3% carbon
and 72.7% oxygen?
What is the empirical formula of a
compound that is 25.9% nitrogen
and 74.1% oxygen?
More to try…
Calculate the empirical formula of
a compound that is 94.1%
oxygen, 5.9% hydrogen.
Calculate the empirical formula of
a compound that is 79.8%
carbon, 20.2% hydrogen.
From there, find it’s molecular formula:
If given the molar mass (how many
g/mol of the compound) then you
can calculate the molecular formula
from the empirical formula.
Take the compound’s empirical
formula mass and compare to the
molecular mass. The molecular
mass will be a multiple of the
empirical formula’s mass.
Example:
If the molecular mass of the first
example (N2O5) problem is 216
g/mol, then what is the
molecular formula for the
compound?
Try these:
What is the molecular formula of a
compound whose molar mass is
60.0 g and whose empirical
formula is CH4N?
What is the molecular formula for
a compound whose molar mass is
78 g and whose empirical
formula is CH?
Obj. 10-11 cont…
• calculate hydrated compounds (bonded to water) same way…
~ use ‘’ to separate water from crystal
~ since water is a compound, you are finding the coefficient…
NOT the subscript!
• practice…
What is the formula of a compound that contains 18.28% Ca,
32.36% Cl and 49.36% H2O?
Ca = 18.28%
= 0.457 moles ***
=1
40
0.457
Cl = 32.36%
35
= 0.925 moles
=2
0.457
H2O = 49.36%
= 2.742 moles
=6
18
0.457
CaCl2  6H2O