Materials Included In Kit

Additional Materials Required

Safety Precautions

Acetic acid is a corrosive liquid. Avoid exposure of all chemicals to eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Have students wash their hands thoroughly before leaving the lab. 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. The product solutions may be disposed of according to Flinn Suggested Disposal Method #26b.

Lab Hints

• The lab work and calculations can easily be accomplished in one 50-minute lab period.
• Students may have a difficult time with pouring the sodium bicarbonate in the bottle and capping it before any carbon dioxide escapes. Have students practice with table salt to perfect the technique before running the procedure with sodium bicarbonate.
• It may be difficult for the students to get the inflated balloon of Part 2 in and out of the 1000-mL graduated cylinder. Two plastic rods may be needed to gently coax the bottle and inflated balloon down and up the cylinder walls. A 2000-mL graduated cylinder may also be employed.

Teacher Tips

• The buoyancy of objects in air is the same principle as objects in water. Review this principle with students. A handout discussion section on mass and buoyancy in air is included, along with a discussion and question section. Make copies to pass out to the students after they answer the Post-Lab Question.

Answers to Prelab Questions

1. Why is no mass lost or gained in a chemical reaction?
   The mass of atoms resides in their nuclei. Chemical reactions only involve the rearrangement of electrons to form and break bonds between these atoms. Since the nuclei never change in a reaction, the total mass does not change.
2. A chunk of steel has a mass of 400 g. Assuming the volume of the chunk is negligible, use Equation 6 to determine what volume a boat made of this piece of steel would have to be in order to just float on the surface of the water. Assume the density of water to be 1.00 g/cm³.
   
   Equation 5 states that for a floating object the measured mass is equal to the volume of the object times the density of water.
   
   Since the boat just floats on the surface, its mass minus volume times density is zero.
   

   0 = m – V_aρ_H2O where ρ_H2O is equal to 1.00 g/cm³ and m = 400 g

   Substituting these values into the equation and rearranging give us
   
   {13304_Answers_Equation_9}
   
   The volume of the boat must be at least 400 cm³ to just float.

Sample Data

Data Table A	Trial
1. Mass of weighing dish and sodium bicarbonate	3.75 g
2. Mass of bottle, acetic acid solution, and cap	46.35 g
3. Total mass before reaction (1 and 2)	50.10 g
4. Total mass after reaction	50.06 g
5. Mass loss or gain (–/+)	–0.04 g
Data Table B
1. Level of water with unreacted assembly submerged. (step 7)	640 mL
2. Level of water without assembly (step 8)	500 mL
3. Mass of assembly before reaction (step 10)	54.64 g
4. Mass of assembly after reaction (step 14)	54.17 g
5. Mass loss or gain (–/+)	–0.47 g
6. Level of water with reacted assembly submerged (step 15)	790 mL
7. Level of water without the assembly (step 16)	490 mL
8. Volume (displacement) of unreacted assembly (1 and 2)	140 mL
9. Volume (displacement) of reacted assembly (5 and 6)	300 mL

Answers to Questions

1. Did the mass of the assembly change after the reaction was complete in either Part 1 or Part 2? If so, propose possible explanations.
   
   For either Part 1 or Part 2, gas could have escaped from the balloon. For part 2, the air can be considered a fluid. Since the volume of the apparatus increased due to the reaction, the buoyant force acting on the apparatus also increased. This in turn would reduce the net force acting on the balance. Less force on the balance would give an apparent lower mass.

Buoyancy in Air

1. Calculate the buoyant force of the inflated balloon. The density of air, ρ_a, at 20 °C, is 1.21 x 10^–3 g/cm. The acceleration due to gravity, g, is 98° cm/sec².
   F_B = (300 cm³) x (1.21 x 10^–3 g/cm³)^–V(3.55 x 10^–3 N)(980 cm/sec²)
   F_B = 355 g•cm/sec² or 355 ergs
2. Use Equation 7 to calculate the volume difference of the unreacted assembly and the reacted assembly. How does this compare to the measured value?
   

   m_m(before) – m_m(after) = [V_a(after) – V_a(before)]ρ_a

   {13304_Answers_Equation_10}
   

   {13304_Answers_Equation_11}
   ΔV  40 cm³ or 140 mL
   The 140 mL is 12.5% less than the measured 160 mL.

Introduction

In a chemical reaction, matter is neither created nor destroyed, but rather is conserved. That is true everywhere, except in this apparent contradictory experiment! Use the discrepant event to learn what we really measure when we “weigh” something on a balance.

Concepts

• Conservation of mass
• Buoyancy
• Mass versus weight

Background

The law of conservation of mass states that the mass of reactants in a chemical reaction equals the mass of the products. Matter is neither created nor destroyed. In this experiment, the reaction of sodium bicarbonate, NaHCO₃, with acetic acid, CH₃CO₂H, will be used to confirm the law. When mixed, the bicarbonate reacts with acetic acid to form carbonic acid. The carbonic acid then decomposes to produce carbon dioxide gas.

In Part 1, the reaction mixture will be contained in a closed bottle. This allows for the measurement of the mass of the reactants and the mass of the products without the loss of any material from the reaction.

In Part 2, the experiment will be modified by replacing the cap with a balloon. The carbon dioxide gas produced will inflate the balloon. This setup is also massed before and after the reaction, like Part 1.

Objects in fluids have two forces acting on them. For instance, in water there is the downward force due to gravity, mg, making the object sink, and an upward force, called buoyancy, which pushes upward on the object. This buoyant force is equal to the volume of the water displaced by an object, V_w, times the density of water, ρ_w, times the acceleration due to gravity, g.

Experiment Overview

In this two-part experiment, the mass of a closed system will be measured before and after a chemical reaction takes place to verify the law of conservation of mass. The same experiment will be performed again, the second time utilizing a balloon to allow the volume of the system to expand when the reaction takes place. The volume and mass of the system will again be measured before and after the reaction. Students will determine the cause of any differences in the mass values determined in Part 1 and in Part 2.

Materials

Prelab Questions

1. Why is mass neither lost nor gained in a chemical reaction?
2. A block of steel has a mass of 400 g. Assuming the volume of the chunk is negligible, use Equation 8 to determine what volume a boat made from this piece of steel would have to be in order to just float on the surface of the water. Assume the density of water to be 1.00 g/cubic centimeter.

Safety Precautions

Acetic acid is a corrosive liquid. Avoid exposure of all chemicals to eyes and skin. Sodium bicarbonate is slightly toxic by ingestion. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Be sure to wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

Part 1. Conservation of Mass

1. Place a weighing dish on the balance. Add approximately 1.5 g of sodium bicarbonate to the weighing dish. Record the mass of the weighing dish and the sodium bicarbonate in Part 1 of the Data Table.
2. Obtain a 120-mL plastic bottle with cap. Add 40 mL of 1 M acetic acid to the bottle.
3. Dry the outside of the bottle with paper towels, then mass the bottle and cap. Record the mass of the bottle, acetic acid and cap in Data Table A.
4. Set the bottle down on the lab bench.
5. Fold the weighing dish with the sodium bicarbonate in half, creating a spout.
6. Hold the folded weighing dish with one hand and the bottle cap with the other.
7. Quickly pour the contents of the weighing dish into the bottle. Immediately cap the bottle tight.
8. The bottle will bulge as gases are produced from the sodium bicarbonate reacting with acetic acid. The cap is strong enough to keep the bottle sealed under pressure.
9. Once the reaction is complete, mass the bottle and the now empty weighing dish together on the balance. Record the total mass in Data Table A.

Part 2. Testing Buoyancy

1. Remove the cap from the bottle and drain the water from it. Rinse the bottle several times and fill the bottle with 40 mL of 1 M acetic acid.
2. Obtain a balloon. Weigh out approximately 1.5 g of sodium bicarbonate in a weighing dish. Fold the weighing dish and carefully pour the sodium bicarbonate into the balloon.
3. Secure the mouth of the balloon around the bottle opening.
4. Fold the balloon against the side of the bottle and wrap a rubber band around the balloon and bottle to hold the balloon in place (see Figure 3).
   {13304_Procedure_Figure_3}
5. Fill a 1000-mL graduated cylinder to the 500-mL mark with water.
6. Place the bottle and balloon apparatus in the graduated cylinder. Use a spatula or pencil to completely submerge the bottle and balloon.
7. Record the level of water with the bottle completely submerged in Data Table B.
8. Slowly remove the bottle from the water. Record the resultant level of water in Data Table B.
9. Dry off the bottle with paper towels.
10. Place the bottle on a balance capable of reading 0.01 g. Record the mass of bottle and balloon in Data Table B.
11. Take the bottle off the balance and remove the rubber band, being careful not to let any of the sodium bicarbonate fall into the bottle.
12. Place the bottle with the balloon back on the balance along with the rubber band.
13. Raise the balloon until a few grains of sodium bicarbonate fall into the bottle. As the pieces dissolve, carbon dioxide gas is released, inflating the balloon. The rest of the sodium bicarbonate now falls into the bottle, further inflating the balloon.
14. When all the sodium bicarbonate has dissolved and the balloon is no longer being inflated, record the mass of the apparatus and rubber band in Data Table B.
15. Remove the bottle from the balance. Submerge the bottle and inflated balloon again in the graduated cylinder (see Figure 4). Record the level of water in the cylinder in Data Table B.
    {13304_Procedure_Figure_4}
16. Remove the bottle and record the level of water in Data Table B.
17. Dispose of the solution in the bottle as directed by the instructor.

Student Worksheet PDF

13304_Student1.pdf