UNDERSTANDING THE MOLE
INTRODUCTION
The relative mass of anything is
actually how many times heavier the object is compared to the lightest
object. The atomic masses of the atoms are all relative masses.
They are relative to the lightest element, which is hydrogen. Although
modern atomic masses are based on the carbon-12 isotope, hydrogen is still
assigned a mass of one and the comparison is still valid for our
purposes. Carbon, which has a relative mass of 12, is actually 12 times
heavier than hydrogen. In this lab you will be dealing with the relative
mass of beans and then you will be asked to draw a parallel to the atomic
masses of the elements.
PURPOSE
To better understand the mole system and the table of atomic
masses by comparison with a model system
MATERIALS
Part 1: 4 different types of beans, paper
cup, balance
Part 2: one mole of each of 5 elements
(sulfur, iron, aluminum, zinc, and copper)
Part 1
PROCEDURE
1.) Zero your balance with the paper cup on it.
2.) Count out exactly 100 beans of one type. Discard any beans
which differ greatly from an "average" bean.
This is important because if you do
not do this, your results will not be accurate. Mass
the beans.
3.) Calculate the mass of one bean by dividing the total mass of 100
beans by 100. Do this for each type of
bean. Record in the data table.
4.) Determine the relative mass of each type of bean by comparing it to
the lightest type of bean. The
calculation should look like
this:
relative
mass = average mass of bean _
average mass of lightest bean
5.) Mass out (do not calculate) the relative mass (in grams) of each kind
of bean and count the beans massed.
(In other words, if the relative mass is 3.4,
then find out how many beans it takes to get a mass of 3.4
grams.) Do not throw away any of the relative mass
piles of beans because you will be asked questions
about them later.
6.) Now calculate the number of beans in one relative mass of each
bean. Do this by dividing the relative mass
by the average mass of one
bean. Number of beans = ___ (one relative mass/average mass of one bean)
Compare this number to the number you got in
step 5.
DATA TABLE
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Kidney |
Pinto |
Navy |
Lentil |
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1.) |
Mass of
100 beans (g) |
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2.) |
Average
mass of one bean (g) |
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3.) |
Relative
mass of beans |
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4.) |
Measured
number of beans in one relative mass |
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5.) |
Calculated
number of beans in one relative mass (round to whole #) |
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QUESTIONS (Part 1)
1.) What did you find out about the number of beans in one relative
mass? Was it the same for each type of
bean or different?
2.) How do your
calculated values compare to your measured values? Were they the same
(within one bean) or
different?
3.) Compare the volume of the relative mass piles. Are they the same or different? Why?
4.) What is the average mass of the lightest bean? What is the relative mass of the lightest bean?
5.) Think very hard
on this one and give a real logical answer. Why are
there always the same number of beans
in the relative mass?
6.) Hydrogen is the
lightest element and each atom has an average mass of 1.66 x 10-24
grams. This is a very
small number, but remember that it is
only one atom. What is the relative mass of hydrogen if it is the
lightest element?
Part 2
Below is a chart reporting the average masses of
individual atoms of different elements. Calculate the relative mass of
each element and record it in the chart. Then look up the atomic mass on
the Periodic Table and record it in the table.
|
Atom |
Mass of one atom (g) |
Relative
mass (to hydrogen) |
Atomic
Mass |
# of atoms in one relative mass |
|
hydrogen |
1.66 x 10-24 |
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sulfur |
5.31 x 10-23 |
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iron |
9.30 x 10-23 |
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aluminum |
4.49 x 10-23 |
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zinc |
1.08 x 10-22 |
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lead |
3.44 x 10-22 |
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copper |
1.05 x 10-22 |
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QUESTIONS (Part 2)
1.) How do the atomic masses found on the Periodic Table compare to the
relative masses you calculated?
Were they the same or different?
2.) What do you know
about the number of atoms of each element in one relative mass? Was it
the same or
different?
3.) For each of the
above elements, find out how many atoms there are in the relative mass you
discovered.
You will have to divide the relative mass in
grams by the mass of one atom in grams. Express your answers
in scientific notation.
4.) So, how many
atoms are there in an atomic mass of any atom if the unit is grams?
(Remember, this number
was not really known for many years. The actual
number is not important, but what is important is that it
was always known that in the same
relative masses, you would have the same number. The number,
however, is equal to one mole.)