016 – The law of conservation of mass

The law of conservation of mass is a fundamental principle in chemistry, first articulated by Antoine Laurent de Lavoisier in the 18th century. It states, “Nothing is lost, nothing is created, everything is transformed.” This principle is verified through chemical reactions by demonstrating that the total mass of reactants equals the total mass of products, provided the system is closed. In this experiment, two chemical reactions will be conducted to validate the conservation of mass.

Educational Goals

  1. Understand chemical reactions: Gain detailed insights into various types of chemical reactions, including gas evolution and precipitate formation, and their role in confirming conservation laws.
  2. Explore experimental integrity: Investigate the importance of maintaining a closed system for accurate experimental validation of mass conservation principles.
  3. Enhance laboratory techniques: Develop proficiency in using scientific tools like balances, graduated cylinders, and reaction vessels to achieve accurate measurements and reliable results.
  4. Encourage curiosity: Foster curiosity through experimentation with alternative reactants and conditions, promoting a deeper understanding of chemical processes.
  5. Critical data analysis: Learn to analyze data critically, identify sources of experimental error, and propose strategies to mitigate these errors for future experiments.
  6. Link theory and practice: Bridge theoretical principles of conservation laws with hands-on laboratory experiences to solidify understanding and applicability in real-world contexts.

Protocol

Part A: The reaction between sodium bicarbonate and acetic acid

  1. Weigh the rubber balloon using the balance.
  2. Using the weighing boat, weigh approximately 2 g of sodium bicarbonate (NaHCO3).
  3. Introduce the weighed powder into the rubber balloon.
  4. Measure 25 ml of acetic acid (CH3COOH) 1M using the graduated cylinder and pour it into the Erlenmeyer flask.
  5. Attach the clamp to the base of the balloon.
  6. Secure the balloon over the opening of the Erlenmeyer flask, taking care not to let the solid fall into the liquid.
  7. Weigh the system (flask, clamp, Erlenmeyer and the two reactants) before the reaction. The mass is found in the results table.
  8. Detach the clamp from the balloon.
  9. By pressing the glass rod against the flask, drop the solid into the Erlenmeyer flask.
  10. Gently swirl the solution until the complete dissolution by rocking the Erlenmeyer flask from left to right.
  11. Attach the clamp to the base of the balloon.
  12. Weigh the system again once the reaction is completed. The mass is recorded in the results table.
  13. Detach the flask (keep the clamp at the base) from the Erlenmeyer flask.
  14. Weigh the flask with the attached clamp.

Part B: The reaction between calcium dichloride and disodium carbonate

  1. Measure 15 ml of calcium chloride (CaCl2) 1M solution with the graduated cylinder.
  2. Pour the solution into the 50 ml beaker #1.
  3. Weigh beaker #1 on the balance.
  4. Measure 15 ml of disodium carbonate (Na2CO3) 1M solution using the graduated cylinder.
  5. Pour the solution into the 50 ml beaker #2.
  6. Weigh beaker #2 on balance.
  7. Pour the contents of beaker #1 into beaker #2.
  8. Stir the contents of beaker #2 with the glass rod.
  9. Weigh the two beakers after the reaction and record the mass in the results table.

Anticipated Outcomes

Part A

  • Step 1. The rubber balloon weighs 7.5g
  • Step 2. The weighing boat weighs 0.5g, but after tare we have 2.2g of NaHCO3.
  • Step 7. Wood clamp = 0.45g, Erlenmeyer 250mL = 200g, balloon = 7.5g. Total devices weight = 207.95g
  • Inside the balloon, there’s 2.2g of NaHCO3 and 1.2g of air for a total of 3.4g
  • Inside the Erlenmeyer, there is 25g H2O, 1.49g CH3COOH, 0.32g of air, and negligible amount of H+ and CH3COO–, for a total of 26.81g
  • Total weight = devices weight (207.95g) + 3.4g + 26.81g = 238.18g
  • Step 12. The reaction will produce some CO2(g), together with CH3COOH, H+, Na+, and HCO3–. The total weight should stay approx. the same, considering some CO2 could escape through the non-hermetic seal between balloon and Erlenmeyer.
  • Step 14. Balloon (7.5g) +wood clamp (0.45g) + air (1.20g) = 9.15g

Part B

  • Step 3. 50mL beaker weight = 75g, 15mL of CaCl2 0.2M = 21.44g for a total of 96.44g
  • Step 6 15mL of Na2CO3 1M = 22.62g, for a total of 97.62g
  • Step 9 The mix of 2 solutions will create a precipitate of CaCO3(s) and the total weight will be 119.06. When removing 75g for the beaker, the substances weight (44.06g) represents the total weights found at steps 3 and 6.

General observations

  • Mass conservation validation: Demonstrate that the total mass of the system remains constant before and after each reaction, confirming the law of conservation of mass.
  • Observable reactions:
    • Part A: Expect to observe vigorous bubbling as carbon dioxide gas (CO₂) is released during the reaction between sodium bicarbonate and acetic acid.
    • Part B: A white precipitate (calcium carbonate, CaCO₃) will form, indicating the chemical reaction between calcium dichloride and disodium carbonate.
  • Data consistency: Experimental results should show negligible differences in mass measurements, with slight variations attributed to equipment sensitivity or minor leaks in the system.
  • Skill development: Participants will enhance their ability to conduct precise measurements, observe reaction dynamics, and draw conclusions from empirical data.
  • Environmental implications: Encourage reflection on the importance of chemical reactions in environmental processes, such as water treatment and industrial applications.

Summary of Assignment by Grade Range

Grades 9-10

  • Focus: Introduce students to the foundational concept of mass conservation in chemistry.
  • Activities:
    • Record initial and final system masses accurately.
    • Observe key reaction indicators like gas evolution and precipitate formation.
    • Complete data tables and answer guided questions about the experiments.
  • Goals:
    • Understand the visible evidence of chemical transformations.
    • Develop basic measurement and observation skills.
    • Relate the experimental results to theoretical principles of mass conservation.

Grades 11-12

  • Focus: Apply and analyze experimental methods to validate conservation laws in a more detailed manner.
  • Activities:
    • Perform stoichiometric calculations to predict expected masses of reactants and products.
    • Investigate experimental discrepancies and propose reasons for any deviations.
    • Explore alternative reactants or experimental conditions for further validation.
  • Goals:
    • Link theoretical knowledge of stoichiometry and reaction types to laboratory practice.
    • Strengthen critical thinking skills by analyzing and interpreting experimental data.
    • Deepen understanding of the law of conservation of mass and its applications.

College-Level/Advanced Students

  • Focus: Conduct an in-depth exploration of reaction mechanisms, system design, and advanced applications of conservation principles.
  • Activities:
    • Design variations of the experiments to test hypotheses about reaction behavior in different conditions.
    • Assess the impact of open vs. closed systems on reaction mass and outcomes.
    • Research real-world applications of mass conservation, such as industrial chemical processes and environmental chemistry.
  • Goals:
    • Refine experimental techniques and evaluate their precision.
    • Explore broader scientific implications of conservation laws.
    • Enhance the ability to critically analyze and present findings in professional contexts.

Laboratory essentials

Instruments

  • Electronic balance
  • Weigh-in basket
  • Rubber balloon
  • Wood clamp
  • Spatulas
  • Glass rod
  • 250mL Erlenmeyer
  • 2x 50mL beakers
  • 25mL graduated cylinder

Products

  • Sodium bicarbonate (powder)
  • Acetic acid 1M (solution)
  • CaCl2 1M (Solution)
  • Na2CO3 1M (solution)
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