Names: _________________________
_________________________
_________________________
Date: ________ Period: ____
Lab: Beanium (Bn)
(Isotopes and Average Atomic Mass)
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Introduction:
The average atomic mass of an element is the weighted average of the masses of the isotopes of that
element. The weighted average takes into account both the mass and relative abundance of each
isotope as it occurs in nature.
Purpose:
In this lab you will determine the average atomic mass of the fictitious element beaniuim, Bn, from a
mixture of three naturally occurring isotopes with different masses.
You will calculate the weighted average atomic mass, which is “weighted” by the different natural
abundances (%’s) of each isotope.
Equipment and Materials Needed:
triple-beam balance
a sample of vegium in a plastic cup
two additional empty plastic cups
one or two stiff cards (for handling vegium)
Procedure:
1)
Sort the isotopes of vegium into separate piles and count the “particles” of each isotope
in your sample. Record the number of particles of each type in the Data Table.
2)
Weigh an empty cup on the balance and record its mass.
Place all of one isotope in the cup and reweigh it. Record that mass also.
Subtract to find the mass of just the isotope.
3)
Using the other two cups, repeat the process for the other two isotopes and record your
results. Be careful: all similar-looking cups do NOT have the same mass.
4)
When you are sure you have gathered all the required data, mix the three isotopes
together again and return them. Return the empty cups to the prep area for reuse.
Check to make sure that no “isotopes” ended up in the sink or on the floor.
5)
Complete the Calculations table to determine the Average Atomic Mass of vegium.
DATA TABLE
Color of
isotope:
____________
____________
____________
Total:
Mass (g):
# of particles:
1
Calculations: (Use the Data Table on page 1 to complete the calculations for the table below)
CALCULATIONS TABLE
Color of
isotope:
a. Average
Mass (g):
b. %
Abundance:
____________
____________
____________
Overall:
= 100 %
Calculations: (Follow these instructions CAREFULLY to complete the calculations for the table above)
a. From the Data Table on the previous page, divide the mass by the # of particles to find the
average mass of the each isotope.
To get the Overall average, Do NOT add the averages, divide the Total mass by the Total # of particles.
b. From the Data Table on the previous page, divide the # of particles by the Total # of particles
and multiply by 100 to get the % relative abundance of each isotope.
c. Calculate the weighted Average Atomic Mass of beanium from its 3 isotopes using the
following formula:
d.
Avg. At. Mass = (%)(Mass1) + (%)(Mass2) + (%)(Mass3)
(Note: the %’s must be in decimal form, that is, 78.5% must be 0.785 or 2.2% must be 0.022)
SHOW YOUR WORK:
Record your answer:
g
2
Questions:
1)
If you compared your answers with those of another lab group and found a difference,
how would you explain that, assuming neither group made a “mistake?”
_____________________________________________________________________
_____________________________________________________________________
2)
If each group did the lab again using a larger sample, would you expect the differences
that time to be larger or smaller than the first time? Why?
_____________________________________________________________________
_____________________________________________________________________
3)
Why isn’t the atomic mass of each element a whole number?
(like Neon: 10 protons + 10 neutrons = 20 atomic mass of neon)
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_______________________________________________________________________
4)
The “mass number” of “an atom” is not on the periodic table because the periodic table
lists the _________________________________ of each element. Explain.
_______________________________________________________________________
_______________________________________________________________________
Conclusion: Write a paragraph discussing the lab’s purpose (on the first page)
and what you observed and learned about the average atomic mass of elements.
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