Unit 3:
Atomic Theory
Law of Conservation of Mass
• Mass is neither created nor destroyed
during chemical or physical changes
• The total mass in the reactants has to
be equal to the total mass of the
products
• Atoms are rearranged
John Dalton
• 1808- proposed an explanation for the
law of conservation of mass
Dalton’s Atomic Theory: (p 66)
1. All matter is composed of extremely
small particles called atoms.
2. Atoms of a given element are identical
in size, mass and other properties: atoms
of different elements differ in size, mass
and other properties
3. Atoms cannot be subdivided, created
or destroyed
Dalton’s Atomic Theory: (con’td)
4. Atoms of different elements combine in
simple whole number ratios to form
chemical compounds
5. In chemical reactions, atoms are
combined, separated and rearranged.
Which aspects of Dalton’s theory are
incorrect?
• Atoms are divisible into smaller particles
• The same element can have different
masses ( isotopes)
• The Modern Atomic Theory maintains
Dalton’s points and accounts for the
corrections stated above.
Law of Definite Proportions
• a chemical compound contains the
same elements in exactly the same
proportion by mass regardless of size of
the sample
• Ex. NaCl 39.34% Na and 60.66% Cl
ALWAYS!
C. Johannesson
Law of Multiple Proportions
• If two or more different compounds are formed of
the same two elements, the ratio of the masses of
the second element combined with a certain mass of
the first is always a ratio of small whole numbers
• Ex. CO 1.0g C to 1.33 g O
• CO2= 1.0 g C to 2.66 g O
• Ratio between the 2 oxygens is two!
C. Johannesson
Law of Multiple proportions continued…..
C. Johannesson
The Mole
• SI unit for the amount of substance
• It’s a counting unit
–1
–1
–1
–1
dozen = 12 units
pair = 2 unit
gross = 144 units
MOLE = Avogadro’s # = 6.02 x 1023
• Why do we need such a huge number
in chemistry?
The Mole in perspective
• If you have a mole of pennies, and
distribute them equally among every
person on Earth, how much $$ would
everyone get?
The Mole in perspective
• If you have a mole of pennies, and
distribute them equally among every
person on Earth, how much $$ would
everyone get?
The Mole in perspective
• 1. One mole of paper would make a stack that would
reach to the moon more than 80 billion times
•
2. One mole of grains of sand would be more than all
of the sand on Miami Beach
•
3. One mole of blood cells would be more than the
total number of blood cells found in every human on
earth
• 4. A one liter bottle of water contains 55.5
moles of water
5. One mole of seconds is about 19
quadrillion years, 4,240,666 times the age of
the earth, or 954,150 times the age of the
universe itself
•
6. One mole of cents could repay the United
States National Debt 86 million times
Avogadro’s Number:
• Italian scientist, Amadeo Avogadro
• Number of particles in a mole is known
as Avogadro’s number = 6.02 X1023
Molar Mass:
• The mass of one mole of pure
substance
• Ex. Lithium = 6.94 g/mol
Molar conversions:
Calculate the molar mass of:
•
•
•
•
Cu
NaCl
Ca2SO4
Al(NO3)3
C. Johannesson
Moles & Mass
For Calculations - Use Dimensional
Analysis – Set up
• What is the mass of 3.0 mols of NaCl?
• How many mols are in 30.0 g of H2O?
C. Johannesson
Moles & Particles
• How many atoms are in 3.0 mols of
H2O?
• How many moles of Aluminum, are in
4.02 x 1023 atoms of Al?
C. Johannesson
Moles & Particles & Mass
• Calculate the mass of 3.7 x 1024
molecules of MgCl2.
• How many atoms are in 100.0g of
Silver?
C. Johannesson
Do Now
• How many particles are in 200g of
Gold?
C. Johannesson
Molarity (M)
• Number of moles of solute in one liter of
solution
• Molarity = (moles of solute/ L of solvent)
C. Johannesson
Percent Composition
• Percent by mass of each element in a
compound.
• % composition of any pure substance is
always constant, whatever the source or
sample size.
• Useful in identifying the characteristic of any
substance.
• Used by investigators from many fields,
Analytical chemistry, forensics etc. to
determine the identity of unknown materials.
Sample Problems
• Calculate the percentage composition of
each element in NaNO3.
C. Johannesson
Answer
•
27.05%Na, 16.48% N, 56.47% O
C. Johannesson
Sample Problem 2
Calculate the percentage composition of
Oxygen in Ag2SO4.
C. Johannesson
Answer
•
69.19% Ag, 10.28% S, 20.52% O
C. Johannesson
Water in a hydrate
• What is the mass percentage of water
in the hydrate CuSO4●5H2O?
• 36.08%
C. Johannesson
Determining Chemical Formulas
• Empirical formulas – consists of the
symbols for the elements combined in a
compound, with subscripts showing the
smallest whole-number ratio of the
different atoms in the compound.
• Molecular Formulas – a multiple of
the empirical formula.
C. Johannesson
Guidelines
• Determine the mass of each element
• Convert into moles
• Divide by the smallest mole or the
elements present
• Determine the ratio
• Write the formula (Usually elements are
listed from left to right on the PTOE)
C. Johannesson
Sample Problems
• A compound is analyzed and found to
contain 36.70% K, 33.27% Cl, and
30.03% O. What is the empirical
formula of the compound?
• A 60.00 g sample of tetramethyllead, a
gasoline additive, is found to contain
38.43 g of lead, 17.83 of carbon, and
hydrogen. Find its empirical formula.
C. Johannesson