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Lab # 18: Counting By Weighing
Chemistry 1
Purpose: Often groups of objects are just too large to count. Picture working in an M&M factory and having to
count each chocolate piece individually to insure every bag of M&M’s packaged contains the same number of
pieces. This is not feasible!! This lab will show you how average mass can be used as a means of counting.
This will help you to understand how the mole and molar mass of elements are used to count atoms and
molecules.
Part I: Relating Mass to Number
Materials
kidney beans
pinto beans
250 mL beakers (2)
1 triple beam balance
Procedure:
1. Measure the mass of a 250 mL beaker using the triple beam balance. Make sure that the triple beam is level
before you begin. Record in Data Table 1.
2. Measure 200 mL of red kidney beans in the massed 250 mL beaker.
3. Measure the mass of the beaker with the kidney beans. Calculate the mass of just the kidney beans. Record
the mass in Data Table 1.
4. Repeat steps 1-3 using a different 250 mL beaker and 200 mL of pinto beans.
5. Measure the mass of 10 kidney beans by placing them directly onto the balance, no beaker required. Record
the mass in Data Table 1. Then measure the mass of 10 pinto beans; record.
6.
Use your measurements to determine the mass of one kidney bean and one pinto bean. (Hint: You need to
determine the average mass.
Data Table 1.
Kidney Beans
Pinto Beans
Volume of beans
Mass of 250 mL beaker
Mass of the beaker + beans
Mass of beans
Mass of ten beans
Mass of one bean (calculated)
Answer the following questions:
1. The volume is the same for the two substances. Why isn't the mass the same?
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2. Do the two beakers contain the same number of objects? Explain why or why not.
3. Describe a way to determine the number of beans in each beaker without counting them.
4. Why is it more accurate to calculate the mass of one kidney bean from ten beans rather than just weighing
one?
5. If you had a beaker of kidney beans with a mass of 50 grams, how many kidney beans would you have?
Show your work.
Part II: Molar Mass (Relating Mass to Moles) Samples 1- 4
Materials
5 vials of elements
1 empty vial
balance
Procedure: Determine masses for Samples 1-4 to the nearest 0.01 g and record in the Data
Tables, lines 1 and 2. Look up the molar masses. Complete lines 4-7, showing all work in
the data table. Show units and watch significant figures.
Sample 1 Data Table
Element: Sulfur
1. Mass of sample + vial, g
2. Mass of vial, g
3. Mass of sample, g
4. Molar mass of sample,
g/mol
5. # moles in sample
6. # atoms in sample
7. # atoms in 1.0 g of
sample
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Sample 2 Data Table
Element: Manganese
1. Mass of sample + vial, g
2. Mass of vial, g
3. Mass of sample, g
4. Molar mass of sample,
g/mol
5. # moles in sample
6. # atoms in sample
7. # atoms in 1.0 g of
sample
Sample 3 Data Table
Element: Aluminum
1. Mass of sample + vial, g
2. Mass of vial, g
3. Mass of sample, g
4. Molar mass of sample,
g/mol
5. # moles in sample
6. # atoms in sample
7. # atoms in 1.0 g of
sample
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Sample 4 Data Table
Element: Copper
1. Mass of sample + vial, g
2. Mass of vial, g
3. Mass of sample, g
4. Molar mass of sample,
g/mol
5. # moles in sample
6. # atoms in sample
7. # atoms in 1.0 g of
sample
Part III – Identity of an Unknown
Procedure: There are 0.14 mole of Sample 5. Calculate the mass of Sample 5. Now calculate the
molar mass (find the ratio of g/mole). From the periodic table, what is Sample 5? SHOW YOUR
WORK.
1. Mass of sample + vial, g
2. Mass of vial, g
3. Mass of sample, g
4. Molar mass of Sample 5
(g/mol)
5. Identity of Sample 5
Answer the following questions (show your work!):
1. What is the molar mass of iron, Fe? How many moles of iron atoms are in 111.8 grams of iron?
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2. What is the molar mass of carbon, C? What is the mass of a 4.35 mole sample of carbon?
3. Every water molecule contains one oxygen atom and two hydrogen atoms. A mole of water
molecules contains one mole of oxygen atoms and two moles of hydrogen atoms. What is the mass
of one mole of H2O?
4. What is the molar mass of CO2?
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