Average Atomic Mass
Name _____________________________________________ Period ______ Date __________________
Pre-lab Questions.
1. Jane Doe earns 85 on her math test. She earns 100 on homework. Her teacher says that assessments
are worth 90% of her grade, and homework is worth 10 %. What is her grade in the class? Show
your work.
2. What is an isotope?
3. If the mass of a proton is 1 amu and the mass of a neutron is 1 amu, what is the mass (in amu) of a
chlorine atom that has 17 protons and 20 neutrons?
4. What is the mass (in amu) of a chlorine atom that has 17 protons and 18 neutrons?
So why is the atomic mass of chlorine 35.45 amu?????? The purpose of this lab is help you understand how
atomic mass is determined. In this lab, the small chocolate candies will act as different isotopes of an
element. But, first we must figure out the mass of each type of candy (isotope).
Procedure
Part A. The Plain Isotopes
1. Weigh a bag of plain chocolate candy.
2. Record the mass in the Data Table.
3. Count the number of candy pieces in your
bag and record it below.
Part B. The Peanut Isotopes
1. Weigh a bag of peanut chocolate candy.
2. Record the mass in the Data Table.
3. Count the number of candy pieces in your
bag and record it below.
Part C. The Peanut Butter Isotopes
1. Weigh a bag of peanut butter chocolate
candy.
2. Record the mass in the Data Table.
3. Count the number of candy pieces in your
bag and record it below.
Data:
Mass of Plain Chocolate Candy (from Part A)
Number of Chocolate Candy Pieces in the bag (from
Part A)
Mass of Peanut Chocolate Candies (from Part B)
Number of Chocolate Candy Pieces in the bag (from
Part B)
Mass of Peanut Butter Chocolate Candies (from Part
C)
Number of Chocolate Candy Pieces in the bag (from
Part C)
Calculate mass of one candy: Mass of candy isotope = Total Mass of bag / Number of Candies in a bag.
Results. Complete the following calculations. As always, you must show all work, show all units in the
appropriate places, observe significant figure rules, and box your answers. Reminder: counted numbers have
unlimited significant figures.
1. Use your data to calculate the mass of one plain chocolate candy. This is the mass of the plain
isotope.
2. Use your data to calculate the mass of one peanut chocolate candy. This is the mass of the peanut
isotope.
3. Use your data to calculate the mass of one peanut butter chocolate candy. This is the mass of the
peanut butter isotope
4. Suppose you are given a mixed sample containing millions of chocolate candy pieces. The sample is
30.00% plain, 58.00% peanut, and 12.00% peanut butter. Use your answers from #1-3 to calculate
the weighted average mass of an M&M.
Imagine you were given a candy but you didn’t know if it were plain, peanut or peanut butter. Your
weighted average is the best number (assumption) that you could always use for the mass.
Questions.
1. In your pre-class questions above you answered that a particular chlorine atom weighs 37 amu.
Why can’t we always use that atomic mass for any chlorine atom?
2. Explain how Question #4 from your Results Section relates to the atomic mass written on the
Periodic Table.
3. List two possible errors in this lab and ways you could eliminate them if you were to do the lab
again. Be specific.
4. Calculate the average atomic mass of chromium. Its composition is: 83.79% with a mass of
51.94 amu; 9.50% with a mass of 52.94 amu; 4.35% with a mass of 49.95 amu; and 2.36% with
a mass of 53.94 amu. As always, you must show all work, show all units, observe significant
figures and box your final answers.