THE CASE OF THE ADULTERATED BAKING SODA
Introduction:
Perhaps you’ve never tasted baking soda. There are some people who use it in place of
toothpaste. It has a very salty taste. Its main use, however, is in baking because it decomposes
to form, among other things, carbon dioxide gas. This gas is responsible for making things like
cake “rise”, giving them that nice fluffy texture.
Because of its salty taste, unscrupulous persons have added a much cheaper compound
(sodium chloride) to the soda, passing it off as pure baking soda. This mixture has suddenly
shown up in your local grocery store. Hundreds of people have phoned in complaining that their
biscuits won’t rise. The store manager has just called the school and asked whether the
chemistry class would be able to analyze this material to ascertain the degree of contamination.
Knowing that you are a very capable chemistry class, the challenge has been handed to you.
Sodium bicarbonate (baking soda) decomposes when heated strongly as follows:
2 NaHCO3 (s)  Na2CO3 (s) + H2CO3 (g)
The H2CO3 further decomposes to form CO2 (g) and H2O (g) as follows.
H2CO3  CO2 (g) and H2O (g)
In this process, carbon dioxide and water are released to the atmosphere as gases.
Therefore, you should be able to determine the amount of baking soda in the mixture by noting
how much mass is lost as you heat the sample. This piece of data in combination with the mass
of the original sample should enable you to determine the percent purity of the adulterated
baking soda. Be sure to record all of the data in the event that the baking soda company might
insist that you appear as an expert witness in this case. Good luck and work carefully.
Purpose:
To determine the percent purity of baking soda which has been contaminated with salt
(sodium chloride).
Equipment/Materials:
To be determined by the student.
Safety:

Goggles should be worn in the laboratory at all times.
Procedure (inquiry):
To be determined by the student.
Show all plans and formatted data tables to your teacher before you actually begin
in the laboratory.
Summing Up:
1. Calculate the percent purity of baking soda from your data. Show all of your calculations.
2. Explain your calculation.
3. Did your sample decrease in mass during the experiment?
Procedure:
1. Record the vial type A., B., or C in box #1.
2. Mass your empty test tube and write the value in box #2.
3. Weigh out an amount very close to 2.000 g of baking soda into your test tube, and enter the
exact mass in box #3.
4. Calculate the mass of the baking soda and enter in box #4.
5. Determine the theoretical yield of Na2CO3 using stoichiometry and enter in box #5.
6. Assemble a ring stand and test tube clamp. Be sure to position the test tube at a 45o angle
and also be sure that the open end of the test tube is not point toward anyone. Position the
clamp near the top of the test tube so that you will have plenty of clearance for the flame of
your Bunsen burner as you heat the calcium carbonate in the test tube.
7. Holding the burner at the bottom, wave the flame back and forth under the solid in the
bottom of the test tube for five (5) minutes.
8. Using a metal test tube holder, transfer the hot test tube to a beaker and allow it to cool for
five (5) minutes.
9. Mass the test tube with the Na2CO3 mixture (it might have some salt also) and enter the value
into box #6.
10. Determine the amount of Na2CO3 mixture produced and enter into box #7.
11. Determine the amount of CO2 and H2O produced and enter into box #8.
12. Use the mixture of CO2 and H2O to calculate the percent of Na2CO3 in your “adulterated”
baking soda sample and enter in box #9.
13. From your instructor, find the true percentage of Na2CO3 for your sample (Vial A, B, or C).
Calculate the percent error for your experimental result and enter in box #10.
14. Calculate the percent of CO2 and H2O in your original sample and enter in box #11.
Data Table:
Vial Type A, B, or C
1.
Mass of the empty test tube
2.
Mass of the test tube and “adulterated”
baking soda
3.
Mass of the “adulterated” baking soda
4.
Theoretical Yield of Na2CO3
5.
Mass of the test tube and mixture
6.
(Na2CO3 and NaCl)
Mass of mixture
7.
(Na2CO3 and NaCl)
Mass of CO2 and H2O
8.
Percent of Na2CO3 in mixture
9.
Percent Error
10.
Percent of CO2 and H2O in mixture
11.
Questions:
1. How can you tell the degree of impurity in your sample?
2. What could you have done incorrectly in the laboratory procedure to cause your data to be
misleading or incorrect? (NOTE: These are known as “sources of error”.)