Molecular and Chemical
Formulas
Atoms combine (bond) to form
molecules.
CHEMICAL FORMULA
• Formula that gives the TOTAL number
of elements in a molecule or formula
unit.
Each molecule has a chemical
formula.
The chemical formula indicates
–which atoms are found in the
molecule, and
–in what proportion they are found.
EMPIRICAL FORMULA
Chemical formula with the smallest
integer subscripts for a given
composition.
Molecular weight
• To get the molecular formula, we
need more information. Need:
molecular weight.
Empirical Formula from Analysis
How do we determine the empirical
formula?
• Start with mass % of elements (i.e.
empirical data) and calculate a
formula, or
• Start with the formula and calculate
the mass % elements.
How to get empirical formulas
C = 12.01 amu O = 16.0 amu
• PERCENT COMPOSITION
(atoms of element)(AW)
× 100
% of element =
FW of compound
% of element =
What is the mass percent of C in CO2?
mass of element
× 100
100 g sample
• MOLAR MASS or FORMULA WEIGHT
sum of atomic weights
The connection between experimental
property (mass) and moles:
FW =12.01+2(16.0)=44.01 amu
%C =
12.01
× 100 = 27.3%
44.01
%O =
2(16.0)
× 100 = 72.7%
44.01
I have 2g of a sample that is 54.2% C by
mass. How many grams of C are in the
sample?
HINT:
If % C in an unknown substance is 54.2%, a 100g sample of
that substance contains 54.2g of C.
(2g of sample)(54 .2g of C)
= 1.08 g of C
100 g sample
C8H18
8C
18H
12.01
1.008
96.08
18.14
114.22
Alternately:
0.542 is the fraction of the sample that
contains carbon.
Sample Problem
Combustion Analysis
• An analysis of nicotine, a poisonous
compound found in tobacco leaves,
shows that it is 74.0%C, 8.65% H and
17.35% N. Its molar mass is 162 g/mol
What are the empirical and molecular
formula of nicotine?
Empirical formulas are determined by
combustion analysis:
4 g sample of an alcohol produces
7.65 g of CO2 and 4.70 g of H2O
upon combustion. What is the
empirical formula of the alcohol?
Structural Formulas
• Using the rules of valence, structural
formulas for simple molecules can be
deduced.
Molecular
formula
NH3
Condensed
molecular
formula
NH3
CO2
CO2
C2H6O
CH3CH2OH
C2H6O
or
ALKANES
Name
Methane
ethane
propane
butane
Pentane
Molecular
formula
CH4
C2H6
C3H8
C4H10
C5H12
In general: CnH2n+2
Condensed
molecular formula
CH3-CH3
CH3CH2CH3
CH3CH2CH2CH3
CH3CH2CH2CH2CH3
CH3OCH3
structural
formula
ISOMERS
Isomers
Isomers are compounds that have the
same molecular formula, but different
structures
Simplest example: butane (C4H10)
2 isomers
CH3⎯CH2⎯CH2⎯CH3
n-butane
CH 3
CH
CH 3
iso-butane
n-pentane
CH 3
iso-pentane
CH 3
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
branched octane (iso-octane)
CH3⎯CH2⎯CH2⎯CH2⎯CH3
CH 2 CH
Straight chained octane (n-octane)
CH 3
pentane has 3 isomers
CH 3
Note: iso-octane also has the same molecular
formula as n-octane (C8H18), but it has a different
structure! (and different properties).
CH3
CH3
C
CH3
CH3
Neo-pentane
CH 3
CH 3
C
CH 2 CH
CH 3
CH 3
CH 3
H
Structural Formulas
• Formulas so far tell us how atoms
are connected to each other
• Lewis structures tell us where the
electrons are.
• What are the shapes of these
molecules?
• Can we use information about
bonding, connectivity of atoms,
molecular formulas, etc. to predict
the shapes (and properties) of
molecules?