Definite Proportions,
Multiple Proportions
and Atomic Theory
Law of
Conservation of Mass



Lavoisier
Mass is neither
created nor
destroyed in a
chemical reaction
If you start with
10g, you will wind
up with 10g
The sum of the masses for the reactants
equals the sum of the masses for the
products
Law of Definite
Proportions
• Proust
• Atoms are neither lost nor gained in a chemical reaction
• Chemical compounds have the same mass ratio of elements no
matter how formed (e.g., water always has an O to H ratio of
8.01:1.00)
• Evidence for this law: Berzelius’ experiments with lead and sulfur
• Constant composition implies constant properties (i.e., water
always boils at 100ºC and freezes at 0ºC)
Chemical compounds have the same mass ratio
of elements no matter how formed
Copper
Carbonate
Law of Definite Proportions
Law of Multiple
Proportions
• Compounds of differing mass ratios of
the same elements are found, but they
will have different properties
• Example: carbon dioxide (CO2) and
carbon monoxide (CO)
Dalton’s Atomic Theory
• John Dalton- (1803)
Law of Multiple Proportions, Atomic
Theory of Matter
Elements of Dalton’s atomic theory:

Matter is composed of atoms

Different elements have different atoms
(e.g, C has a mass=12, O has a mass=16)

Compounds form when different elements combine
in fixed proportions
Determining the
Mass of a Compound

EXAMPLE:

If C has a mass of 12 and O has a mass of 16,
what is the mass of:


a) Carbon monoxide (CO)
 12 + 16 = 28
b) Carbon dioxide (CO2)
 12 + 2(16) = 44
Determining the
Mass of a Compound


Find the mass of NaCl, NaOH, CaCO3, and
H2SO4
The atomic masses for each element are
found on the periodic table.
(for now use these amounts)
Na=23
Cl=35
O=16
Ca=40
C=12
H=1
and S=32