LESSON
10
What’s in a Mole?
Molar Mass
OVERVIEW
Key Ideas
Lesson Type
Lab:
Groups of 4
Chemists compare moles of substances rather than masses because moles are
a way of counting atoms. When considering toxicity, or other properties of a
substance, it is usually the number of particles that matters. The molar mass
of a compound depends on the molar masses of the elements that make up the
compound. The mass of a substance can be converted to moles using molar mass
values from the periodic table.
As a result of this lesson, students will be able to
calculate the molar mass of compounds
describe the approximate magnitude of a mole of a substance
complete simple conversions between mass and moles
Focus on Understanding
The mole is a difficult unit for students to grasp. This lesson assists by having
students create tangible molar samples.
What Takes Place
In this lab, students measure out a mole of several different substances, from
water to aluminum, to gain a conceptual grasp of the quantity of a mole and to
compare moles of different substances. In Part 1 of the worksheet, students figure
out the molar mass of some common substances. In Part 2, they work in groups
to create molar samples of several substances.
Materials
student worksheet
nickels (25)
aluminum cans (14)
2 g iron nails (40), or about 56 g of nails
table salt, 100 g
water, ∼250 mL
balance (1 per station)
beaker or cup for weighing water
weighing boat or beaker to measure salt
50 mL graduated cylinder for water
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Living By Chemistry Teacher Guide
Unit 4 Toxins
Setup
Before class, set up five weighing stations, one substance at each station: nickels,
aluminum cans, water, table salt, and iron nails. You might want to set up more
stations to avoid congestion. In addition, you might want to have 1-mole samples
of each substance set aside for comparison. The amounts listed for the substances
in the materials list are purposefully greater than 1 mole for each substance. This
is to prevent giving away the amount needed for 1 mole. You can adjust according
to your needs.
Cleanup
Recycle materials, or save for reuse.
Section II
Lesson 10 What’s in a Mole?
73
LESSON
10
What’s in a Mole?
Molar Mass
GUIDE
Engage (5 minutes)
Key Question: How can you convert between mass and moles?
ChemCatalyst
Consider 12 nickels, 2 empty aluminum cans, and a balloon full of carbon
dioxide gas.
a. Which has the greatest mass?
b. Which has the greatest number of atoms?
c. Which has the greatest number of moles of atoms?
Explain the reasoning behind your answers.
Sample Answers: Students can consult the periodic table to determine molar masses. Carbon
dioxide has a molar mass of 44.01 g/mol; aluminum has a molar mass of 26.98 g/mol; nickel has
a molar mass of 58.769 g/mol. However, if you don’t know the mass of each sample, the masses
cannot be converted to moles or atoms.
Discuss the ChemCatalyst
➠ Stimulate a discussion on the relationship among mass, moles, and numbers
of atoms.
Sample Questions
What did you base your predictions on?
How could you find out which set of objects has the greatest mass? the
greatest number of moles of atoms?
How can you convert between mass and moles?
What makes predictions about carbon dioxide different from predictions about
nickel or aluminum?
Explore (15 minutes)
Introduce the Lab
➠ Pass out worksheets. Briefly explain the procedure. Students can do
Part 1 individually.
➠ Students work in groups of four on Part 2.
74
Living By Chemistry Teacher Guide
Unit 4 Toxins
LESSON
10
What’s in a Mole?
Name
Date
Molar Mass
Period
LAB
Part 1: Molar Mass
1. Use a periodic table to complete the second column in the table. You
will complete the last column in Part 2.
Chemical formula
Molar mass
g/mol
Mole of what?
Equivalent to:
Cu(s)
63.55 g
Cu atoms
50 ft of 20-gauge copper wire
O2 (g)
32.00 g
O2 molecules
22.4 L oxygen gas at STP
Ni(s)
58.69 g
Ni atoms
12 nickel coins
Al(s)
26.98 g
Al atoms
2 aluminum cans
H2O(l)
18.02 g
H2O molecules
18 mL water
He(g)
4.00 g
He atoms
22.4 L helium gas at STP
NaCl(s)
58.44 g
NaCl units
Hg(l)
200.6 g
Hg atoms
14.7 mL mercury
Fe(s)
55.85 g
Fe atoms
28 nails*
C12H22O11(s)
342.3 g
sugar molecules
0.75 lb sugar
_1
4
cup salt
*Answer will change according to size of nails.
2. How many copper atoms are in 63.55 g of copper?
602 sextillion copper atoms
3. Copper has a larger molar mass than aluminum. Explain what this means.
Individual copper atoms are heavier than individual aluminum
atoms. If you have the same number of both kinds of atoms, the
copper sample will have more mass.
4. Which contains more moles, 1.0 g Al(s) or 1.0 g Hg(l)? Explain your thinking.
One gram of aluminum has more moles than one gram of
mercury. Aluminum atoms are lighter, so it takes more aluminum
atoms to make up a gram.
5. Explain how you determined the molar mass of sugar. Show your work.
Determine the molar mass of each type of atom in the compound.
Then multiply by the number of atoms of each type.
C: 12.01 g/mol; H: 1.008 g/mol; O: 16.00 g/mol.
12(12.01) + 22(1.008) + 11(16.00) = 342.3 g/mol
Living By Chemistry Teaching and Classroom Masters: Units 4–6
© 2010 Key Curriculum Press
Section II
Unit 4 Toxins
Lesson 10 • Worksheet
29
Lesson 10 What’s in a Mole?
75
6. Explain why the table says “NaCl units.”
Sodium chloride is an ionic substance. It is not found as molecules
or individual atoms.
7. What do you think the volume of a mole of carbon dioxide gas would be? Explain
your reasoning.
As we learned in Unit 3: Weather, 1 mole of any gas at STP
occupies the same volume, 22.4 L.
Part 2: Mole Challenge
Procedure
1. There are weighing stations around the room. With your group, visit the stations and
create 1-mole samples of the items found at each station. These samples must be close
to a mole, but they do not have to be exact.
2. Bring your mole samples back to your desk. Be ready to show them to your teacher
and to explain how you determined the amount of each substance.
3. Enter the amounts in the last column of the table in Part 1.
Questions (Show your work.)
1. How many moles of aluminum do you need to make a six-pack of cans?
Two cans are equivalent to about 1 mole of aluminum, so six cans
require about 3 moles of aluminum.
2. How many iron nails do you need in order to have 223.40 g of iron atoms?
223.40 g is four times 55.85 g. If 28 nails weigh 55.85 g, then you
need four times that, or 112 nails.
3. What volume of water do you need in order to have 25 moles of H2O molecules?
If 1 mole of water is equivalent to approximately 18 mL, then you
need 25 times 18 mL, or 450 mL of water, to have 25 moles.
4. Making Sense Suppose you have 1 g of sugar and 1 g of water. Which one has more
molecules? Explain how you arrived at your answer.
A gram of water will have more molecules. A water molecule has
much less mass than a sugar molecule, so it takes more molecules
to make a 1 g sample.
5. If You Finish Early The average teenager drinks 868 cans of soda per year.
Determine how many aluminum cans per year this represents for your school by
estimating the number of students. How many moles of aluminum atoms does
this represent?
Answers will vary depending on the school population. For a school
of 1,000 students, the number would be 868,000 cans per year.
It takes about two aluminum cans to make 1 mole of aluminum
atoms. Therefore, the number of aluminum cans should be
divided by 2 to get the number of moles. In this case, the answer is
434,000 moles of aluminum atoms.
30
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Unit 4 Toxins
Lesson 10 • Worksheet
Living By Chemistry Teacher Guide
Living By Chemistry Teaching and Classroom Masters: Units 4–6
© 2010 Key Curriculum Press
Unit 4 Toxins
Explain and Elaborate (15 minutes)
Discuss the Molar Mass of Compounds
Sample Questions
What do you need to know in order to figure out the mass of a mole of any
compound on the planet?
How could you figure out the molar mass of carbon dioxide, CO ?
2
Why is the molar mass listed for oxygen in the periodic table different from the
molar mass of oxygen gas?
Key Point
You can figure out the mass of 1 mole of any element or compound using a
periodic table. For compounds, you must sum the molar masses of all the
atoms in a molecule or a formula unit to find the molar mass of the compound.
For example, each molecule of water contains two atoms of hydrogen and one
atom of oxygen. The molar masses of hydrogen and oxygen are 1.008 g/mol and
16.00 g/mol, respectively. So the molar mass of water, rounded to the correct
significant digits, is 18.02 g/mol.
Compare a Mole’s Worth of Different Substances
➠ You might want to display a mole of each substance used for the lab.
Sample Questions
What does 1 mole of water look like? What is its volume? (1 mole is 18 g,
which has a volume of 18 mL when liquid—about the amount of an ice cube.)
What volume, in milliliters would 10 mol of water occupy? (180 mL)
How many molecules of water are in your 1 mol sample? How many atoms of
aluminum are in a 1 mol sample?
Which substance occupies the greatest amount of space per mole of substance?
(any of the gases) the least? (the densest substance)
What is the volume of 1 mole of any gas at STP? (22.4 L)
Which has more mass, 1 mole of oxygen molecules, O2, or 1 mole of aluminum
atoms, Al? (oxygen molecules)
Key Points
A mole of atoms or molecules of a solid or a liquid is an amount you usually can
hold in your hand. The mass per mole of the different elements in the periodic
table ranges from 1.008 g/mol for hydrogen atoms to 238.0 g/mol for uranium
atoms. Two aluminum cans contain about one mole of aluminum atoms. It is a
convenient amount of substance to work with.
A mole of any gas, if it is at standard temperature and pressure, always has
a volume of 22.4 L. Recall that this is Avogadro’s law. So a mole of oxygen
molecules occupies the same volume as a mole of helium atoms or a mole of
chlorine gas molecules. A mole of a gas occupies a fairly large space—equivalent
to ten 2 L soda bottles—because there is so much space between gas molecules.
This is why it is difficult to compare the volume of a mole of a gas, like oxygen,
and the volume of a mole of a solid, like aluminum.
Section II
Lesson 10 What’s in a Mole?
77
Wrap-up
Key Question: How can you convert between mass and moles?
The molar mass of a compound is the sum of the atomic masses of the atoms
in the compound.
The molar mass of a compound allows you to convert between moles of the
compound and grams of the compound.
One mole of a solid or a liquid is an amount that you usually can hold in
your hand.
One mole of a gas occupies a volume of 22.4 L. This is larger than the volume
of 1 mol of a liquid or a solid because of the space between gas molecules.
Evaluate (5 minutes)
Check-in
You have 1 mol of oxygen molecules, O2, and 1 mol of carbon dioxide
molecules, CO2. Which has more mass? Which has a larger volume at
room temperature?
Answer: CO2 has a larger molar mass. Its molar mass is 12.02 2(16.00), or 42.02 g/mol. The
molar mass of O2 is 2(16.00), or 32.00 g/mol. They have equal volumes at room temperature.
Homework
Assign the reading and exercises for Toxins Lesson 10 in the student text.
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Living By Chemistry Teacher Guide
Unit 4 Toxins