Calculating Average Atomic Mass Lab
Name____________________
During this lab you will carefully measure and record data which will enable you to
perform the calculations to determine the average atomic mass of a fictitious element.
Obtain a sample of the fictitious element that contains three varieties of dried beans.
Treat each variety of bean as an isotope of the same fictitious element. Separate the three
isotopes into groups labeled A, B, and C, and measure the mass of each isotope. Count
the number of atoms in each sample (each bean represents an atom of the element).
Follow the directions under Analysis to complete the data table below. Apply significant
figure rules, and show work for calculations when completing the data table.
Data Table:
A
B
C
Totals
Total mass
(grams)
Number
Average mass
(grams)
Relative
Abundance
Percent
Abundance
Relative Mass
Review Significant Figure Handout and/or section 1.5-1.6 of your textbook before you
complete the data table. What is an exact value? Give an example of an exact value.
How do exact values affect our calculations? How many times should you round when
completing a multi-step calculation? When should rounding take place and how do I
determine what place value to round my answer to?
Analysis: Using the experimental data, record the answers to the following questions.
1. Calculate the average mass of each isotope by dividing its total mass by the number of
particles of that isotope. Record your data in the data table.
2. Calculate the relative abundance of each isotope by dividing its number of particles by
the total number of particles.
3. Calculate the percent abundance of each isotope by multiplying the relative abundance
by 100.
4. Calculate the relative mass of each isotope by multiplying its relative abundance by its
average mass.
5. Calculate the weighted average mass of all bean particles by adding the relative
masses. The weighted average mass is the atomic mass of your element.
6. Explain the difference between percent abundance and relative abundance. What is
the result when you total the individual percent abundances? the individual relative
abundances?
Post Lab: Word-Processed
1) Use the information from data table to draw a cell or box from the periodic table.
Make up a name and symbol for the fictitious element, being careful not to use a symbol
that is already in use.
2) How many neutrons does H-1, H-2, and H-3 have? What do H-1, H-2, and H-3 have in
common? Name these isotopes and give an alternate symbol for them. Research H-3 and
summarize your findings. What is heavy water and how is it used? Cite your sources.
3) Does your average mass match the average mass for one of your “isotopes”? If so,
which one? If not, why not?
4) Which isotope A, B, or C, had the heaviest mass? Which isotope had the lightest
mass? What type of bean was used for isotope A, B, and C?
5) If the isotopes in this lab were truly isotopes (not dried beans), what characteristics
would they have in common?
6) How do scientists analyze a sample of an element to determine the percent abundance
of each isotope when they cannot count, weigh, and separate each individual isotopes as
we did with the beans? Research this and find out how it’s done. Cite your source.