Understanding the Mole
Bean Lab (Relative Mass)
Introduction:
The relative mass of an object is how many times more massive the object is than a standard object.
The atomic masses of atoms are all relative masses. They can be considered relative to any particular
element. Historically, both oxygen and carbon have served as the reference standard. For our
purposes we can also consider atomic masses relative to the least massive element--hydrogen, with an
atomic mass of approx. one. Fluorine, with a relative mass of 19, is 19 times more massive than
hydrogen, etc. In this laboratory exercise you deal with the relative masses of beans. Then you will be
asked to draw a parallel to the atomic masses of elements.
Target:
Students are able to determine the relative mass of different small particles by comparison with an
arbitrarily chosen mass. This will develop an understanding of the mole concept and molar masses of
elements through an analogy with a model system.
Pre-lab assignment:
1. Read the entire lab
2. Print out the lab (data section to the end of the lab).
3. Below the target, answer the following questions regarding the lab:
a. Explain the process you will follow to figure out the mass of one bean.
b. Why should you calculate the mass of one bean rather than measuring one bean?
c. What should you record before you add the beans to the beaker?
d. In your own words, write down what is meant by “relative mass.”
Procedure:
Part I. Be sure to use the same balance for the entire activity.
1. In a beaker, count out exactly 100 beans of one type. Discard any beans that differ greatly
from an average bean. If you fail to do this, your results will not be accurate.
2. Put a beaker on the balance. Find the mass of the beaker. This you will need to subtract later.
Add the 100 beans to the beaker; record their mass to the nearest 0.01 g.
3. Each team member is responsible for one type of bean provided.
Calculations:
1. Calculate (do not weigh) the mass of one bean of each type. Record the value in the data table.
[Note: "Calculate" means to take the total mass of 100 beans and divide by 100 rather than
weighing one particular bean.]
2. Determine the relative mass of each type of bean by comparison to the lightest type of bean.
Record these values in the data table.
Relative = Avg. Mass of Bean
Mass
Avg. Mass of Lightest Bean
3. Calculate the number of beans in one relative mass of each bean. Do this by dividing the
relative mass by the average mass of one bean. Number of beans is = ( one Relative
mass/Average mass of one bean)
4. Check your calculated results by following these steps: Weigh out the relative mass of each
kind of bean (in grams) and count the number of beans weighed.
a. Place the plastic cup on the balance pan and add beans of your type until the balance
contains one relative mass of your type bean. (don't forget to subtract the weight of
the cup)
b. Count the beans. Record this as the measured number of beans in one relative mass.
c. Pour the beans into a pile. Retain your separate piles of relative masses of beans. You
will answer questions about them later. (Each team member is responsible for his or her
type bean.)
Data:
Analysis and Concept Development:
Lima
Mass of 100 beans (g)
Average Mass of one bean
(g/bean)
Relative mass of beans
(g/”bunch”)
Calculated number of
beans in one relative mass.
Round to the nearest
whole number
(beans/”bunch”)
Measured number of beans
in one relative mass
Pinto
Black
Lentil
Questions:
1. If a “bunch” is defined as the number of beans in 1.00g of the lightest bean, how many beans
are in a “bunch”?
2. What statement can you make about the number of beans in a relative mass of each type of
bean?
3. What statement can you make about the mass of a bunch of beans whose relative mass is:
a.
3.5
b. 2.6
c. 17
4. Calculate the number of “bunches” in 1,000 beans.
5. If you wanted 1,000 of the heaviest beans used in this experiment, how could you measure out
the approximate number without counting?
6. Calculate the number of beans in:
a. 15 bunches of pinto beans.
b. 3.0 g. of a bean whose relative mass is 1.2.
7. What would be the mass of 35 bunches of the heaviest bean?
8. What did you find out about the number of beans in one “bunch” (relative mass)? Where they
the same or different?
9. How do your calculated values compare to your measured values? Where they the same (within
one bean) or different?
10. Compare the volume of the relative mass piles (think about you piles of 100 beans). Where
they the same or different... Why?
a. What is the average mass of the lightest bean?_____________g.
b. What is the relative mass of the lightest bean?_____________g
c. Among the elements, hydrogen has the least massive atoms, an atom of hydrogen has an
average mass of 1.66 x 10-24 g. This is very small, but remember it is only one atom!
What is the relative mass of hydrogen if it is the least massive element?
_____________g
Part II.
Below is a chart reporting the average masses of individual atoms. Calculate the relative mass of each
element and record it in the chart. Then look up the molar mass (atomic mass) of each element on a
periodic table and record it in the table.
Atom
Hydrogen
Carbon
Iron
Aluminum
Zinc
Lead
Copper
Mass of one atom
(g)
1.66 X 10-24
2.00 X 10-23
9.30 X 10-23
4.49 X 10-23
1.08 X 10-22
3.44 x 10-22
1.05 X 10-22
Mass relative to
hydrogen
Atomic Mass
Number of atoms in a
relative mass in grams
1. How do the atomic masses found on the periodic table compare to the relative masses you
calculated?
2. What did you find out about the number of atoms of each element in one relative mass?
3. One mole of atoms contains how many atoms?
4. How many atoms are in one mole of uranium atoms?
5. How many grams are in one mole of uranium atoms?
Conclusion:
Remember this section is where you report your results this is the MEAT of your write up and you
need to have 3 paragraphs.
1st paragraph - you will draw conclusions. Restate you results! Give a valid conclusion based on the
correct interpretation of your results and explain your results reflecting back on the target (what
did you do in this lab? What did you find?).
2nd paragraph - you will evaluate procedure(s) and results including limitations, weaknesses or
errors. (what worked well or what did not work)
3rd paragraph - you will Identify weaknesses and state realistic suggestions to improve the
investigation. (if you were to change this lab…. what would you change so it works better?)