Lab: Percent Error & Average Atomic Mass
Purpose: This activity is designed to introduce you to determining how much error can occur when we try to
quantify chemistry with measurements. We will soon want to start answering questions such as:
o
o
o
o
o
-How much of a chemical is present?
-What mass of a chemical is required to react?
-How many liters of a chemical can react?
-What is the percent error in this lab?
-What is the average atomic mass for M&M’s
A measurement can be compared with its accepted value by calculating its percent error. The percent
error is found from the following formula:
% error = Accepted value – Experimental Value x 100
Accepted Value
Procedure:
1. Each person should obtain 1 bag of “isotopes” (M&M’s).
2. Analyze your individual bag contents and complete the following chart:
Red
Orange
Yellow
Green
Blue
Brown
Total # of
M&M’s
How
many
M&M’s of
each
color
3. Now, combine your results with your lab partner to find the total number of each color M&M for
your lab set. This is your EXPERIMENTAL DATA.
Red
Orange
Yellow
Green
Blue
Brown
Total # of
M&M’s
TOTAL
NUMBER
of M&M’s
for your
lab
group
According to the M&M/Mars Company, M&M bags are filled according to a specific color distribution.
Color blends were selected by conducting consumer preference tests, which indicate the assortment of colors
that pleased the greatest number of people and created the most attractive overall effect. On average, here’s
how the mix of colors is distributed:
Plain M&M’s
Brown…….….20.%
Yellow…….….20.%
Red…………….20.%
Orange……….15.%
Green………..15.%
Blue……………10.%
Each large production batch is blended precisely to those ratios and mixed thoroughly. However, since
the individual packages are filled by weight on high-speed equipment and not by count, it is possible to
occasionally have an unusual color distribution.
Also, because of the difference in the sizes of the individual candies, the number of pieces per package
varies. However, the package should be the indicated weight.
Based on knowing the percentage of each color that should be in plain M&M bag, calculate how many
of each color your group should have. In other words, if between you and your lab partner you had a total of
67M&M’s, 20% of them should be brown. In order to calculate how many brown M&M’s you should have, take
20.0% of 67 (0.20 x 67 = 13.4  13 brown M&M’s). You will put all of your answers in correct significant
figures (your percentages tell you how many sig figs to use). Complete the chart by calculating how many of
each colored M&M’s you should have. These are your ACCEPTED VALUES.
This X This
Red
20.%
Orange
15.%
Yellow
20.%
Green
15.%
Blue
10.%
Brown
20.%
TOTAL
NUMBER OF
M&M’s for
your lab
group
Now, calculate the percent error for each color of M&M. Remember: the experimental data is how
many M&M’s of each color you have sitting in front of you, while the accepted value is the number of M&M’s
that the M&M/Mars candy company says should be in the sample. Show your work for each color in the
data table.
Red
Orange
Yellow
Green
Blue
Brown
Work for
calculating
% error goes
here 
Answer for
% error goes
here 
You will now be calculating the average atomic mass for each isotope (color) of M&M’s. Just like the
periodic table you cannot simply add up all the M&M isotopes and divide by the number you added. Instead we
need to find the % abundance (how often you can find each isotope in the world) for each M&M isotope. You
will then mass each M&M isotope. The next step would be to change the % to a decimal and multiply that
decimal by the mass. After you add up all the masses and check for significant figures you will have the
average atomic mass for your M&M element.
Red
Orange
Yellow
Green
Blue
Brown
Red
Orange
Yellow
Green
Blue
Brown
Using your
total
number of
M&M’s from
your lab
group find
the % of
each color
here 
Using your
M&M’s from
your lab
group find
the mass of
each color
here 
Red
Answer

Orange
Yellow
Green
Blue
Brown
Average Atomic
Mass for M&M’s
goes here
Change %
to a
decimal
then
multiply
by mass
goes here

Analysis Questions: You must show all work in order to receive full credit!
1. What is percent error and why is it important to calculate?
2. A 1-lb. bag of M&M’s contains 600 pieces of candy. According to the M&M Company, how many green
M&M’s would you expect in the bag?
3. A bag of Skittles contains 58 pieces of candy. Thirteen are purple. If the Skittles candy company states
that 20% of all their candies are supposed to be purple, calculate the % error in this bag of candy.
4. Calculate the average atomic mass for silicon if 92.21% of its atoms have a mass of 27.98 amu, 4.7%
have a mass of 28.98 amu, and 3.09% have a mass of 29.97 amu.