Section 9.5
THE LAWS
GOVERNING
FORMULAS AND
NAMES
Scientific Laws
• A scientific law is a statement of a relationship
that is invariable so far as it is known.
• The law of definite proportions and the law of
multiple proportions (which we are about to
learn) are invariable statements governing the
formation of compounds.
Historical link
• Remember that originally scientists did not
know about atoms. We cannot see them.
• However, scientists were able to measure
masses.
• In the early 1800s, many scientists were
studying chemical reactions by carefully
measuring the masses of reactants and
products.
Historical Link
• The French scientist Joseph Proust
demonstrated that samples of
copper carbonate always contain 5.3 parts by
mass of copper to 4 parts of oxygen and 1 part
of carbon.
• The principle embodied in this discovery
became known as Proust’s Law and later as the
law of definite proportions.
Law of Definite Proportions
• In samples of any one chemical compound, the
masses of the elements are always in the same
proportions. (Because atoms combine in
simple whole-number ratios, their proportions
by mass must always be the same.)
• Example: Use the models to
determine the formulas for the
two compounds:
Law of Definite Proportions
• In what other ways can you tell
from the pictures that the two
compounds are different?
• When water and hydrogen peroxide
are broken down into their
constituent elements (hydrogen and oxygen)
the mass ratios are different:
• Water has 1 gram H for every 8 grams of O
• Peroxide has 1 gram H for every 16 grams O
Historical Link
• Proust’s experiments inspired
John Dalton to think in terms of atoms.
• With atoms in mind, John Dalton designed
experiments that soon led to the law of
multiple proportions, as well as to Dalton’s
atomic theory.
• The law of multiple proportions can be
described in different ways.
Law of Multiple Proportions
• In your textbook: Whenever the same two
elements form more than one compound, the
different masses of one element that combine
with the same mass of the other element are in
the ratio of small whole numbers.
• My preference: When two elements form two
different compounds, the mass ratio of the
elements in one compound is related to the
mass ratio in the other compound by a small
whole number.
Realplayer\Multiple Proportions.flv
Example:
• Both of these compounds
contain 1 part sulfur (32.06 g).
• Sulfur dioxide has 32.00 g of
oxygen combining with it,
while sulfur trioxide has 48.00 g
of oxygen combining with it.
• The mass ratio of oxygen in these two
compounds is 32 g in SO2 to 48 g in SO3. Note
that this ratio (32:48) simplifies to the small
whole number ratio 2:3.
Naming Compounds Flowchart
SiO2
HBr
MnCl3
Li3PO3
Ba(OH)2
Writing Formulas Flowchart
Hydrogen
cyanide
Potassium
chromate
Sulfur
tetrafluoride
Copper(I)
chloride
Is that everything?
• The binary molecular compounds constitute
only a small fraction of molecular compounds.
• An enormous number of compounds containing
principally carbon, hydrogen, oxygen, nitrogen
and sulfur are classified as organic chemicals.
• A different, more complex but precise system of
naming makes it possible for large and complex
molecules to be precisely identified. This would
be covered in an organic chemistry class.
Anything else?
• Some chemicals can exist in a dried out form
(called “anhydrous”) as well as in a form
where they combine with water molecules
(called a “hydrate”).
• Copper(II) sulfate is one example. It is
white in its anhydrous form (CuSO4),
but turns blue when it is hydrated (it
combines with 5 water molecules to
form the hydrate, 𝐶𝑢𝑆𝑂4 ∙ 5𝐻2 𝑂).
References
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http://reich-chemistry.wikispaces.com/file/view/200pxJoseph_Louis_Proust.jpg/174499761/200px-Joseph_Louis_Proust.jpg

http://www.basicfamouspeople.com/pictures/3214.jpg

http://www.youtube.com/watch?v=3dWdMqZ2UOU

http://www.t3portal.org/T3_Portal_v1/!SSL!/WebHelp/enju_wang/Wange_powerp
oint_ppt1_16_01_rot.jpg

http://img1.photographersdirect.com/img/13429/ps1104605.jpg