MRHS
Chemistry
Y. Varma-Kumar
Name: ________________________________________________ Date: _____________ Pd: __________ Day: ________
VEGIUM LAB
Average Atomic Mass
Objectives
 Determine the weight of each isotope of the fictitious element VEGIUM.
 Determine the relative abundances of the isotopes of vegium.
 Calculate from experimental data the atomic mass of vegium.
Introduction
Isotopes are atoms of the same atomic number having different masses due to different
numbers of neutrons. The atomic mass of an element is a weighted average, taking into account
both the mass and relative abundance of each isotope of that element as it occurs in nature.
The relative abundances and masses of small atomic particles are measured in the laboratory by
an instrument called a mass spectrometer. The mass spectrometer separates particles by mass
and measures the mass and relative abundance of each type of particle it studies. From these
data a weighted average is calculated to determine the atomic mass of the element.
Purpose
In this lab you will carry out experiments and perform the necessary calculations to determine
the atomic mass of vegium. Three isotopes of vegium have been observed in nature: beans, peas,
and black-eyed peas. These isotopes are different varieties of vegium having different masses.
Unlike real isotopes, the individual isotopic particles (beans, peas, and blackeyed peas) differ
slightly in mass, so you will determine the average mass of each type of isotopic particle. Then
you will determine the relative abundance of each isotope, and use this data along with the
isotopic masses to calculate the weighted average mass, or atomic mass, of vegium.
Materials & Equipment
a sample of vegium
electronic balance
weighing boat (to act as a weighing dish)
Experimental Procedure
Carry out the following steps and record your results in your data table. Show all calculations
neatly on a separate sheet of paper. Report all numbers to four significant figures.
1. Weigh all the beans, then all the peas, then all the blackeye peas. Record your data.
2. Count all the beans, then all the peas, then all the black-eye peas. Record your data.
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3. Divide the total mass of each isotope by the number of each isotope to get the average mass
of each isotope.
Note: In all rows except the average mass in the table, the numbers in the "total" column can
be obtained by adding the numbers across each row. Step 3 is an exception because it does
not take into account the fact that there are different numbers of each type of particle.
Rather than adding across, calculate this number using the same method you used to
calculate the other numbers in step 3.
4. Divide the number of each isotope by the total number of atoms, and multiply by 100 to get
the percent abundance of each isotope.
5. Divide the percent abundance from step 4 by 100 to get the relative abundance of each
isotope.
6. Multiply the relative abundance from step 5 by the average mass of each isotope to get the
relative weight of each isotope.
7. Add the relative weights to get the average mass of all particles in vegium, the atomic mass.
8. Return your first sample of vegium in its ziploc bag. Obtain a second sample of vegium and do
the experiments to determine its atomic mass.
Data Table (a part of your Results section)
Beans
Trial
Trial
1
2
Peas
Trial
Trial
1
2
B.E. Peas
Trial
Trial
1
2
Total
Trial
Trial
1
2
Mass of Each Isotope
Number of each Isotope
Average mass of each
“atom”
Percent Abundance
Relative Abundance
Relative Weight
Atomic Mass Trial 1: ________________ Atomic Mass Trial 2: ________________
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Analysis Questions:
Use your experimental data and your knowledge of atomic structure and isotopes to answer the
following questions:
1. Which of your data (in the table) can be measured and which had to be calculated?
2. What is the difference between percent abundance and relative abundance?
3. What is the result when you total the individual percent abundances? The individual
relative abundances?
4. What is the difference between the average mass you calculated in step 3 and the
relative weight you calculated in step 6?
5. Can atomic masses be calculated the way the total for row 3 was calculated? Explain why
your results using this method would or would not be the same as those you obtained by
using step 7.
6. Explain any differences between the atomic masses of your two samples of vegium.
Explain why the differences would be smaller if larger samples were used.
7. In what ways does this lab succeed in imitating “real-life” isotope calculations? In what
ways does it do a poor job at simulating real-life?
8. Compare your results for the atomic mass of vegium to the class results. Explain why the
calculated atomic masses might vary.
9. Identify two possible sources of experimental error and explain which data were most
impacted and how.
10. Identify two possible sources of human error and explain data which were most impacted
and how.
11. Explain why the atomic mass of an element reported on the periodic table is a weighted
average based on relative abundance instead of a straight average not accounting for the
abundance of each isotope.
12. The beanium isotope of vegium is naturally radioactive and decays with a half-life of 33.8
seconds. Starting with one thousand atoms, calculate the number of beanium atoms that
are present every 33.8 seconds for 304.2 seconds. (304.2 seconds is 9 half-lives.) Use a
graphing program to create a table of time(s) and number of beanium isotopes for this
time period.
13. Use a computer graphing program to graph the decay of the beanium isotope where the
number of atoms of beanium remaining depends on the time elapsed.
14. If the actual average atomic mass of Vegium is 0.2050 g, determine percent error.
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