Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Make a glass wool “paint brush” for each student lab group. Wearing gloves, cut small pieces of glass wool and tape a piece to the end of each straw.
2. Fold the pieces of cardboard accordion-style (see Figure 4). Place one folded sheet on each balance as a test tube holder to prevent the tube from rolling off the balance. Enough cardboard is included in the kit to make three cardboard stands.
   {14099_Preparation_Figure_4}
3. Set up a support stand and clamp with a Bunsen burner for each lab station.

Safety Precautions

Universal indicator solution is alcohol-based and is a flammable liquid. When heating sodium bicarbonate, use a borosilicate glass test tube. Inspect the test tube for chips and cracks before use and handle the hot test tube using a test tube clamp. Use a cool flame to heat the test tube. Do not handle glass wool with bare hands. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. After completely cooling, the test tube of sodium carbonate may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab.
• Be sure the test tubes used are borosilicate glass. Inspect the test tubes for chips or cracks before using.
• A burning splint may be used to test for CO₂ in the test tube; however, students may think the water vapor is what causes the flame to be extinguished. The universal indicator solution is a better test to show the gas in the tube is a nonmetal oxide.
• Remind students to use the same balance each time.

Teacher Tips

• Do not show them the actual mass on the balance, just tell them their score. If time permits, you may allow students a second attempt if they are not satisfied with their grades. This second attempt may cost them one point, and they may end up with a lower score, so tell them to only try the second attempt if they are fairly sure they can correct whatever mistake they may have made the first time.

Answers to Prelab Questions

1. What is the molar mass of sodium bicarbonate?

   NaHCO₃ = molar mass of sodium bicarbonate = (22.99 + 1.01 + 12.01+ 3(16.00)) = 84.01 g/mol

2. The decomposition of hydrogen peroxide (H₂O₂) yields water and oxygen gas. If 15.5 g of hydrogen peroxide decomposes, what mass of oxygen gas will be produced?

   2H₂O₂ → 2H₂O + O₂

   {14099_PreLabAnswers_Equation_2}

Sample Data

Time when the heating of the test tube started: ___1:39 pm___
Predicted mass of final product and test tube (g):___34.88 g___
Actual mass of final product and test tube (g):___34.85 g___
Final mass of product only measured (g):___3.44 g___

Answers to Questions

1. What color change occurs, if any, when the brush with the universal indicator solution is inserted into the test tube (step 9)?

   The indicator solution changes from green to yellow-orange.

2. The chemical formula of sodium bicarbonate is NaHCO₃. What common oxide is being produced in the test tube?

   Carbon dioxide, CO₂

3. What do you observe in the upper half of the test tube (step 10)?

   Condensation is occurring.

4. What common substance appears to be a second product of this reaction?

   Water, H₂O

5. The third product of the reaction is sodium carbonate. What is the correct formula for sodium carbonate? Hint: Remember to balance the charges.

   Na₂CO₃

6. Write the balanced chemical equation for the reaction that took place in the test tube. Note: Check with your instructor before balancing to make sure the products are correct.

   2NaHCO₃(s) → H₂O(g) + CO₂(g) + Na₂CO₃(s)

7. Starting with the mass of NaHCO₃ you started with in the test tube (see the data table), use stoichiometry and your balanced equation to calculate the mass of sodium carbonate you should have in the test tube. Show your calculations.
   {14099_Answers_Equation_3}
8. Assuming all the baking soda you started with has been converted into sodium carbonate (with the product gases driven off), what should the test tube and contents weigh now?

   3.47 g Na₂CO₃ + 31.41 g test tube = 34.88 g Na₂CO₃ + test tube

9. Observe the sodium carbonate left in the test tube; compare and contrast the product to some fresh sodium bicarbonate. Record your observations.

   Both are solids in powder form. The sodium carbonate has a slightly grayish tint compared to the sodium bicarbonate, which is whiter.

10. How would the final results be affected if the test tube had not been heated long enough? Explain.

    If the test tube had not been heated long enough, the decomposition reaction would not have gone to completion, and the mass of the test tube and contents would be higher than predicted.

11. CO₂ is denser than air. Why did the CO₂ produced from the reaction rise upward and out of the mouth of the tube?

    The CO₂ was heated, causing it to expand and become less dense than the surrounding cooler air.

12. Using the original mass of NaHCO₃, determine the mass of H₂O produced in this reaction. Show your calculations.
    {14099_Answers_Equation_4}
13. Using the original mass of NaHCO₃, determine the mass of CO₂ produced in this reaction. Show your calculations.
    {14099_Answers_Equation_5}
14. Add the two masses from Questions 12 and 13 along with the calculated mass of Na₂CO₃ from Question 7. What is the total mass of products in this reaction?

    0.59 g H₂O + 1.44 g CO₂ + 3.47 g Na₂CO₃ = 5.50 g total

15. How does the mass from Question 14 compare with the initial mass of sodium bicarbonate you put in the test tube? Explain why this makes sense.

    The masses are the same. This agrees with the law of conservation of mass; any chemical reaction leaves the total mass unchanged, regardless of the extent to which other properties are changed.

16. Using your data from lab, calculate the percent error for the sodium carbonate measured at the end of lab. Show all work.
    {14099_Answers_Equation_6}

References

Special thanks to Bob Becker, Kirkwood High School, Kirkwood, MO, for sharing this activity with Flinn Scientific.

Introduction

Predict the mass of your product correctly to earn your grade! In this lab, stoichiometry and a balanced chemical reaction will be used to determine the masses of the products. How close your prediction is to the final mass weighed will determine your group’s grade. Stay on target and be sure to utilize your best lab etiquette to achieve the top grade!

Concepts

• Stoichiometry
• Law of conservation of mass
• Balanced chemical equation
• The mole

Background

Through observations and careful lab work, the chemical reaction of the decomposition of sodium bicarbonate will be observed in this lab activity. Starting with sodium bicarbonate as the single reactant, the products will be predicted and balanced. Once a balanced chemical reaction is found, stoichiometry will be used to predict the mass of the products. Stoichiometry is the branch of chemistry that deals with the numerical relationships of reactants and products in chemical reactions.

After predicting your final mass, you will receive a grade from your instructor on how close your prediction was to the correct amount.

Knowing your grade, you will be able to calculate the percent error for the reaction. The percent error for a reaction can be used to show how far off your results are from the theoretical mass.

Experiment Overview

The purpose of this activity is to utilize stoichiometry and chemical reactions to predict the final product and mass. How close your group is to the correct value will determine your grade. Stay on target!

Materials

Prelab Questions

1. What is the molar mass of sodium bicarbonate?
2. The decomposition of hydrogen peroxide (H₂O₂) yields water and oxygen gas. If 15.5 g of hydrogen peroxide decomposes, what mass of oxygen gas will be produced?

Safety Precautions

Universal indicator solution is alcohol-based and is a flammable liquid. When heating sodium bicarbonate, use a borosilicate glass test tube. Inspect the test tube for chips and cracks before use and handle the hot test tube using a test tube clamp. When using the Bunsen burner, tie back long hair and do not wear loose, long sleeves. Never leave lit burners unattended. Do not handle glass wool with bare hands. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines.

Procedure

1. Obtain a large borosilicate test tube and weigh it on one of the balances. Record this mass in the Data Table on the Target Stoichiometry Lab Worksheet. Note: Borosilicate is a type of glass that can be subjected to very high (and low) temperatures without shattering.
2. Go back to your lab station and add 4–5 g of baking soda (sodium bicarbonate, NaHCO₃) to the test tube.
3. Using the same scale as before, weigh the test tube with the baking soda. Record this mass in the data table.
4. Calculate the mass of the baking soda in the test tube and record the amount in the data table.
5. Holding the test tube nearly horizontal, shake the baking soda gently so that it spreads out a bit (see Figure 1).
   {14099_Procedure_Figure_1}
6. Tighten the test tube clamp securely around the test tube, just below the lip so that it is positioned nearly horizontally about 20 cm above the lab desk (see Figure 2).
   {14099_Procedure_Figure_2}
7. Light a burner and adjust it to a large cool flame hitting the bottom half of the test tube as shown in Figure 2. Record the time you started heating on the Target Stoichiometry Lab Worksheet. Note: This heat will initiate a chemical change (a decomposition reaction) that breaks the NaHCO₃ down, not into its elements but into three separate compounds.
8. Put one drop of the green universal indicator solution on the small “paint brush.” Note: The “bristles” of the brush are made of glass wool. Do not handle with bare hands.
9. Carefully insert the end with the universal indicator into the mouth of the test tube (see Figure 3). See if you can observe a distinct color change. If CO₂ is being produced, it will create an acidic environment and turn the drop yellow or even orange. Answer Questions 1 and 2.
   {14099_Procedure_Figure_3}
10. Look carefully at the upper half of the test tube. Answer Questions 3 and 4.
11. Move the burner occasionally to a different spot to ensure a thorough heating of the entire bottom half of the test tube.
12. Consider the substance that is left in the test tube. It may look just like the baking soda you put in the test tube, but it has actually been converted into something else—sodium carbonate. Answer Question 5, keeping an eye on the time.
13. After you have been heating the test tube for 8–10 minutes, turn off the burner and let the test tube cool for 5–6 minutes.
14. While waiting for the test tube to cool, answer Questions 6–9.
15. If your test tube has been cooling for 5–6 minutes, it should be ready for the official weigh-in! Bring the test tube, along with the worksheet containing your prediction from Question 8, to the instructor who will weigh it on the same scale you used before. Record your final mass. Your grade will be based on how closely your prediction came to the actual mass (see the Scoring Table).
16. After you have finished all of the above, rinse the test tube out into the sink.
17. Answer the Post-Lab Questions.
18. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

14099_Student1.pdf