EMPIRICAL VS.
MOLECULAR FORMULA
•
Define molecular formula and empirical formula.
• Find the empirical formula given the molecular formula.
• Find the molecular formula given the empirical formula &
the molecular mass.
Aim: How can we determine the molecular
formula from the empirical formula?
Key Words:
ions, molecules,
simplest whole
number ratio,
molecular mass,
empirical formula,
molecular formula,
percent composition
Essential Questions:
What is the difference between
empirical and molecular formulas?
2. In order to find the molecular
formula of an unknown compound,
what two units of information do
you need?
3. How do you calculate the empirical
formula from the molecular formula
and vice versa? Use examples to
explain your answer.
1.
Do Now
What is the molecular
formula for
hydrogen peroxide?
(Show the steps of the
crisscross method)
What is the formula for
magnesium oxide?
(Show the steps of the
crisscross method)
How Do We Find the Atomic Ratios of
Molecular Compounds?
Empirical Formulas
Empirical Formula – indicates the simplest (smallest) whole
number ratio in which the atoms of the elements are
present in a compound (even if the formula is not supposed
to be reduced as is the case for covalent compounds).
Molecular Formula – indicates the actual whole number
ratio in which the elements are present in a compound.
All formulas for ionic compounds
are empirical formulas!
Find the Empirical Formula for Glucose
from you knowledge of its molecular
formula
Find the Empirical Formula for the
Following Compounds
C2H6
Ethane
C5H12
Pentane
CH4
Methane
C2H4
Ethene
C5H10
Pentene
Now that we know how to write convert a
molecular formula  empirical formula,
can we do the reverse:
empirical  molecular?
If so, what additional information do you
think we would need to know?
4.
A compound has an empirical formula of
CH3 and a molecular mass of 30 amu.
What is its molecular formula?
Learning Check
1. Which of the following are empirical formulas?
Circle all that apply!
A. C4H10
B. Na2SO4
C. N2O5
HC2H3O2
‘
7. What is the molecular formula of a compound
whose empirical formula is CH4 and molecular
mass 16?
A. 10
B. 40
C. 51
D.78
Learning Check
2.
The empirical formula of a compound is CH2. The
molecular formula of this compound could be
A. CH4
3.
D. C3H8
B. N2O3
C. N2O
D. N2O5
The molecular mass is 42 amu. What is the empirical
formula?
A. CH
1.
C. C2H4
What is the empirical formula for the compound
dinitrogen tetroxide?
A. NO2
6.
B. C2H2
B. CH2
C. CH3
Determine X for the empirical formula CH2
A. CXH14
B. C32HX
D. CH4
Learning Check
4.
The empirical formula of a compound is CH. It’s molecular
mass could be
A. 10
5.
B. 40
C. 51
D.78
What is the molecular formula for problem 4?
A. 10
B. 40
C. 51
D.78
8.
The simplest formula for Vitamin C is C3H4O3. Experimental
data indicates that the molecular mass of Vitamin C is 176
amu. What is the molecular formula for Vitamin C?
1.
Determine the empirical formula for:
A. C6H6
B. C2H2
C. CH4
D. CCl4
Learning Check
1
Which of the following are empirical formulas?
A. C4H8
B. CH3COOH
C. NH3
D. C7H16
9. A formula has an empirical formula of ClCH2 and
molecular mass of 98g/mole. What is the molecular
formula?
10. Oxalic acid has the empirical formula HCO2 and a
molecular mass of 90g/mole. Find the molecular
formula of oxalic acid.