Chemical
Composition
Counting by Weighing
Atomic Mass
The Mole
Molar Mass
Percent Composition
Formulas of Compounds
Calculating Empirical Formulas
Calculating Molecular Formulas
Counting by Weighing
Counting Atoms by Weighing
The Mole
Objectives:
1. Understand the
mole concept and
Avagadro’s
number
2. Convert between
moles, mass and
atoms
Molar Mass
Objectives:
1. Understand the
definition of
molar mass
2. Convert between
moles and mass
of a chemical
compound
Percent
Composition
Formulas of
Compounds
Calculating
Empirical
Formulas
Definition of Empirical Formula
Empirical Formula – 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
Solving for Empirical Formulas
•
•
•
•
Always assume 100.0 g sample
Change percents to grams
Calculate moles of each element
Divide moles by smallest mole amount to
determine ratio
• Multiply to get whole numbers (if
necessary)
Example: An oxide of aluminum is formed by the reaction
of 4.151 g of aluminum with 3.692 g of oxygen.
Calculate the empirical formula for this compound.
Example: When a 0.3546 g sample of vanadium
metal is heated in air, it reacts with oxygen to
achieve a final mass of 0.6330 g. Calculate
the empirical formula of this vanadium oxide.
Example: A sample of lead arsenate, an insecticide used
against the potato beetle, contains 1.3813 g of lead,
0.00672 g of hydrogen, 0.4995 g arsenic, and 0.4267
g of oxygen. Calculate the empirical formula for lead
arsenate
Calculating
Molecular
Formulas
Objective: calculate
the molecular formula
of a compound given
its empirical formula
and molar mass
Example: A white powder is analyzed and found to have
an empirical formula of P2O5. The compound has a
molar mass of 283.88g. What is the compounds
molecular formula?
Putting it all together…
What is the empirical formula of a
hydrocarbon that produces 2.703 g CO2 and
1.108 g H2O when combusted?
• What is the empirical formula of a
substance containing carbon, hydrogen,
and oxygen if 1.000 g of substance
produces 1.467 g CO2 and 0.6003 g H2O
upon combustion?
• The molar mass of the substance is 120
g/mol. What is the molecular formula?
• What is the molecular formula of a
substance containing carbon, hydrogen,
and oxygen if it has a molar mass of 234
g/mol and 0.360 g of substance produces
0.406 g CO2 and 0.250 g H2O upon
combustion?
Next Exam
Multiple Choice
10/22/09
Free Response
10/23/09