Do Now – April 7(3), 2009
1. What is the molar mass of cobalt?
2. What is the mass of one mole of
silicon?
3. Calculate the molecular weight of
oxygen gas, O2.
Agenda
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Do Now/Announcements
Review Molecular Weight
Percent Composition
Bubble Gum Investigation
Exit Ticket
TOMORROW IS EARTH DAY!
Objectives
• SWBAT calculate the percent composition of
given compounds.
• SWBAT perform an investigation to
calculate the percent composition of bubble
gum.
Percent
You already know this stuff!
How do you calculate percent?
(Part/Whole) x 100
Example
There are 46 girls and 54 boys at the
movie theater. What percent is boys?
(54/100) x 100 = 54%
Percent
You already know this stuff!
How do you calculate the grade on your
test?
(Points Correct/Total Points) x 100
Example
You got 56 points correct out of 70 possible points.
What percent of the test did you get correct?
(56/70) x 100 = 80%
(MASTERY LEVEL)
Percent Composition is the same thing!
...Except you use chemical formulas
• A chemical formula is a
combination of chemical symbols
and subscript numbers used to
represent a compound
H2O
CO2
Why is percent composition so
important?
• Chemists often develop new
compounds for industrial,
pharmaceutical, and home
uses
• After a synthetic chemist
produces a new
compounds, an analytical
chemist analyzes the
compound to provide
experimental proof of its
composition and its
chemical formula
Percent Composition
• It is the analytical chemist’s job to identify the
elements a compound contains and determine
their percent by mass
Hi, I’m Carrie. I am
analyzing a new compound that
my buddy Germaine just
created. A 100-g sample
contains 35 g of element X and
65 grams of element Y. I can
calculate the percent
composition using a handy
formula!
Percent Composition
Here it is!
mass element
x 100 = percent
mass compound
So…
35 g element X
x 100 = 35% X
100 g compound
65 g element Y
100 g compound
x 100 = 65% Y
Percent Composition
You see…
Percent composition is
just the percent by mass
of each element in a
compound.
Your teacher told me to
tell you to add this to
your Vocabulary Log?
Percent Composition of that beloved
substance….
WATER!
We already know the chemical formula – H2O
mass element x 100 = % element
mass compound
mass hydrogen x 100 = % hydrogen
mass water
mass oxygen x 100 = % oxygen
mass water
Percent Composition of that beloved
substance….
WATER!
We already know the chemical formula – H2O
mass element x 100 = % element
mass compound
2(1.0 g) H x 100 = 11% H
18.0 g H2O
1(16.0 g) O x 100 = 89% O
18.0 g H2O
Percent Composition
Calculating percent composition is just one
more step in describing compounds.
Imagine you find two intriguing substances
during your Australian study abroad trip in
college. Natives tell you that both
substances contain iron and oxygen - one is
iron(II) oxide, FeO, and the other is iron (III)
oxide, Fe2O3. They also tell you that the
substance with more iron is worth a lot of
money. They ask you to help them figure out
which substance has more iron.
PERCENT COMPOSITION IS THE WAY!
1.
2.
3.
4.
Steps for Calculating Percent
Composition
Molecular weight (total mass of
compound)
Mass contributed by each element
Calculate the percentage of each
element
Check! Add up all percents to
make sure they add up to 100%
Percent Composition of the
Australian Iron-Oxygen Substances
iron(II) oxide, FeO
iron (III) oxide, Fe2O3
Which is worth more GREEN?
Percent Composition with
Polyatomic Ions
Calculate the percent composition of
barium nitrate
Percent Composition Practice
• Complete Practice Problems 42 –
45 on page 331 in your book.
• Use the examples we have worked
through as well as Example
Problem 11-10 on page 330 if you
need help.
Percent Composition
Bubble Gum Investigation
Your task is to calculate the
percent composition of
bubble gum. We are
assuming that bubble gum
is composed of 2 things –
gum and sugar. You should
be able to calculate the
percent gum and percent
sugar after completing
your investigation.
Exit Ticket
1. What is the formula for calculating
percent composition?
2. Calculate the percent composition
of sulfur trioxide.
S’more Lab
• What makes up the delicious goodness of a
s’more?
• Well, as a chemistry student, you can find
out just what makes a s’more taste so good!
• It’s all about the ingredients – the parts of
the whole!
• Your mission: calculate the percent
composition of a s’more?