Transcript Document

Law of Definite Composition and
Law of Multiple Proportions
Chemistry
Mrs. Coyle
Joseph Proust (France 1799)
Law of Definite
Proportions:
A given compound
always contains elements
in a certain proportion by
mass. (Constant
composition).
Atoms combine in whole number ratios, so
their proportion by mass will always be the
same.
Example: H2O is always made up of 2
atoms of H and one atom of O.
The ratio of O to H in water is always 16:2 or
8:1.
Example:
KCl always contains one atom of K for
every one atom of Cl
In KCl, potassium and chlorine always
have a ratio of “39.09 to 35.45” or “1.1 to
1” by mass.
Law of Multiple Proportions (John Dalton)
When the same two elements combine to
form more than one compound:
the ratios of the mass of one element in
the first compound to its mass in the
second compound, (as it combines with
the same mass of the other element), can
always be expressed as ratios of small
whole numbers( ex: 1:3 or 2:5).
Example of Law of Multiple Proportions
 Carbon combines
with oxygen to form
CO and CO2 .
Mass of
Carbon(g)
CO
12.01
Mass of
Ratio of O
Oxygen(g) in CO2 to O
in CO
16.00
CO2
12.01
32.00
2:1
Practice Problem 1
In the carbon compounds ethane (C2H6)
and ethene (C2H4), what is the lowest
whole number ratio of H atoms that react
with the same number of C atoms?
Answer: 3:2
Practice Problem 2
Carbon reacts with oxygen to form two
compounds as shown:
Compound A: 2.41 g C, 3.22 g O.
Compound B: 6.71 g C, 17.9 g O.
Find the lowest whole number ratio of C
that react with an equal mass of O.
Solution Strategy
1. Find the grams of carbon per 1 g of
oxygen for each compound.
2. Divide the highest answer by the
smallest answer.
Answer: 2:1
Practice Problem 3
Lead forms two compounds with oxygen as
shown:
Compound A: 2.98 g Pb, 0.461 g O.
Compound B: 9.89 g Pb, 0.763 g O.
For a given mass of oxygen, what is the lowest
whole number mass ratio of lead in the two
compounds?
Answer: 2:1