Name:
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Date:
Lab # 6: Average Atomic Mass of Halloweenium and Half-Life of Cubium
Accelerated Chemistry
Objective
1. To practice manipulating the formula for calculating the average atomic mass of an element
2. To gain a better understanding the term percent abundance
3. To gain a better understanding of the random, yet quantifiable decay of an element by
simulating its radioactive decay and determining the value of its half-life
Introduction
In Part A of this investigation, you will be determining the average atomic mass of a timely new
element, Halloweenium, which bears an uncanny resemblance to Halloween candy. Although usual
tests on a new element typically require fancy instrumentation, you will be doing more basic tasks,
namely counting, massing and calculating. From the mass number and percent abundance of each
isotope of Halloweenium, you will calculate the average atomic mass.
In Part B, you will construct a plot showing the radioactive decay of another new element, cubium. In
any sample of a radioactive isotope, the individual atoms are decaying in a random fashion. It is
impossible to predict which atom is the next to decay, yet statistically you can predict how many atoms
will decay in a certain period of time. Scientists measure how much time elapses while half of the
atoms of a given radioactive sample decay. That time is called the half-life. To simulate radioactive
decay, every two minutes (nominally) you will determine the amount of decayed and undecayed
Cubium atoms. Once the rate of decay is known, it is possible to calculate the half-life.
Pre-lab Questions
1. Explain what is meant by the term half-life.
2. Suppose you have a radioactive isotope with a half-life of two years and you start with 800 grams
of this substance today.
a. How much will you have two years from today?
b. How much will you have eight years from today?
3. Is the quantity of a radioactive isotope ever equal to exactly zero? Explain your answer.
4. How will you determine the percent abundance of each isotopic form of Halloweenium?
5. Why must you calculate the “average” mass of each isotope before calculating the average atomic
mass of the element?
AccelLab6-AAMHalf-life
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PROCEDURE - PART A. HALLOWEENIUM
1. Remove the candies from the zip-lock bag. Record bag number on Data Table #1.
2. Sort the candies by type. Record the name of each different isotope and the number of each
isotope.
3. Calculate the percent abundance (by count, not by mass) of each isotope. Record.
4. Using the triple beam balance, determine the total mass of each isotope by massing all of the atoms
of the isotope together. Record.
5. Calculate the average mass of each isotope; record.
6. Replace the candies in the bag and return the candies and the balance to the area designated by
your teacher. Clean your work area and wash your hands before you leave the laboratory.
PROCEDURE - PART B. CUBIUM
1. Count the initial "atoms" of "Cubium". Place in the paper bag and shake up and down and splill
the “atoms” onto the bench top.
2. Remove any atoms with the colored face up; they represent decayed Cubium. Record the number
of decayed atoms; calculate the number of remaining atoms.
3. Every two minutes (approximately), repeat steps 1-2. (Don’t worry if it takes more or less than 2
minutes to shake, remove and count.)
4. Continue this process until either one Cubium or no Cubium atoms remain.
5. Repeat the process, but start with half the atoms of Cubium.
DATA - Average Atomic Mass of Haloweenium
Table 1. Halloweenium Data (Use correct significant figures!)
Isotope (candy
type)
# Atoms of
Isotope
Percent
abundance
(calculation)
Total Mass of
Isotope, g
Totals
Calculation
Average Mass of
Isotope, g
(calculation)
NA
(Exact #’s)
Sig Figs!
1. Using the percent abundance and average mass of each isotope, calculate the (weighted) average
atomic mass of the element “Halloweenium”.
AccelLab6-AAMHalf-life
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PART B. Half-Life of Cubium
Table 2. Cubium Decay Data
“Time”,
min
0
#
Decayed
Cubium
0
#
Undecayed
Cubium
100
#
Decayed
Cubium
0
#
Undecayed
Cubium
50
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
26.0
28.0
30.0
32.0
34.0
36.0
38.0
40.0
42.0
44.0
46.0
48.0
50.0
52.0
54.0
56.0
58.0
60.0
62.0
64.0
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Analysis and Conclusions
1. Graph your data; let x = time, y = number of cubium atoms remaining. Use the same graph for
both cubium tries (100 and 50 cubium atoms). Draw a “best-fit line” for each curve.
2. Describe the appearance of your graph. How does the graph for decay of 100 cubium atoms differ
from that for 50 cubium atoms?
3. Theoretically, what percent of the cubium atoms should be removed with each shake?
4. What was the half life of cubium you observed when you started with 100 atoms? 50 atoms?
Would you expect the same half life with 50 cubium atoms? Explain.
5. a. Is it possible to identify in advance which cubium atoms will decay? Explain.
b. Is it possible to predict how many cubium atoms will decay with each shake? Explain.
Synthesis Show your work!
1. Since the average atomic mass of hydrogen is 1.0079 and hydrogen is composed of one proton and
one electron then why isn’t the average atomic mass of Helium exactly 4.0316 (4 × 1.0079)?
Helium has an average atomic mass of 4.0026.
AccelLab6-AAMHalf-life
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2. A chemist wishing to do an experiment requiring 47Ca (half life = 4.5 days) needs 5.0µg of the
nuclide. What mass of 47Ca must be ordered if it takes 48 h for delivery from the supplier?
3. The first atomic explosion was detonated in the desert north of Alamogordo, New Mexico, on July
16, 1945. What fraction of the strontium-90 (t1/2 = 28.8 years) originally produced by that
explosion still remains as of July 16, 2004?
4. At a flea market, you’ve found a very interesting painting done in the style of Rembrandt’s “dark
period” (1642-1672). You suspect that you really do not have a genuine Rembrandt, but you take
it to the local university for testing. Living wood shows a carbon-14 activity of 15.3 counts per
minute per gram. You painting showed a carbon-14 activity of 15.1 counts per minute per gram.
Could it be a genuine Rembrandt? Show your calculation! (C-14 t1/2 = 5,730 yrs)
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