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Lab: Conservation of Mass

  1. What is the law of conservation of mass?


  2. Why is it hard to prove the law when a gas is produced?


  3. What is the difference between an “open” system & a “closed” system?


    Reaction #1: Sodium Bicarbonate &1M Acetic Acid

    Materials: 2 beakers, balance, acetic acid, sodium bicarbonate, plastic bag

    NaHCO3 & CH3COOH:

    Open System Starting mass: ___________ g

    Ending mass: ____________ g Amount Changed: ________ g

    NaHCO3 & CH3COOH:

    Open System Starting mass: ___________ g

    Ending mass: ____________ g Amount Changed: ________ g

    Part 1: Open System

    1. Calibrate, or set the balance to 0.

    2. Fill a beaker with mL of 1 M acetic acid.

    3. Add of sodium bicarbonate into the second beaker.

    4. Place both beakers on the balance & record the starting mass.

    5. Dump the baking soda into the beaker. Do not stir.

    6. Place the empty beaker back on the balance. Record the ending mass, including both beakers.

    7. Calculate the amount of mass changed.

      NaHCO3 & CH3COOH :

      Closed System Starting mass: ___________ g

      Ending mass: ____________ g Amount Changed: ________ g

      NaHCO3 & CH3COOH :

      Closed System Starting mass: ___________ g

      Ending mass: ____________ g Amount Changed: ________ g

      Part 2: Closed System

    8. Clean & dry both beakers.

    9. Fill a clean beaker with 30 mL of 1M acetic acid.

    10. Add one spoonful of sodium bicarbonate into a clean plastic bag.

    11. Gently place the beaker with vinegar in the plastic bag. Do NOT spill the acetic acid!

    12. Try to push all air out of the bag. Seal the bag & place it on the balance without spilling the acetic acid. Record the starting mass.

    13. Without opening the bag, tip the beaker, mixing the vinegar with the sodium bicarbonate.

    14. Still without opening the bag, record the ending mass of the contents of the plastic bag.

    15. Calculate the amount of mass changed.


Analysis:

  1. State the reactants & products of this reaction.

    The reactants are _____________________ & _____________________.

    The products are _____________________, _____________________ & _____________________.


  2. Compare part 1, the open system, to part 2, the closed system. What was the same? What was different?

    Reaction #2: Water & Effervescent Tablet

    Water & Effervescent Tablet

    Open System Starting mass: ___________ g

    Ending mass: ____________ g Amount Changed: ________ g

    Water & Effervescent Tablet

    Open System Starting mass: ___________ g

    Ending mass: ____________ g Amount Changed: ________ g

    Materials: 1 Erlenmeyer flask, balance, water, 2 effervescent tablets, balloon

    Part 1: Open System

    1. Confirm that the balance is still calibrated.

    2. Fill a clean Erlenmeyer flask with 50 mL of water.

    3. Place the flask & an effervescent tablet on the balance & record the starting mass.

    4. Place the tablet into the flask of water. Swirl & wait 3 minutes.

    5. Once the reaction is complete, record the ending mass.

    6. Calculate the amount of mass changed.

      Water & Effervescent Tablet

      Closed System Starting mass: ___________ g

      Ending mass: ____________ g Amount Changed: ________ g

      Water & Effervescent Tablet

      Closed System Starting mass: ___________ g

      Ending mass: ____________ g Amount Changed: ________ g

      Part 2: Closed System

    7. Thoroughly clean the flask & fill with 50 mL of water

    8. Place an effervescent tablet into a balloon. You may need to break the tablet in half.

    9. Place the balloon around the rim of the flask, but do not let the tablet fall into the water.

    10. Find & record the starting mass of the flask & balloon with tablet.

    11. Lift the balloon, causing the tablet to fall into the water. Swirl & wait 3 minutes.

    12. Once the reaction is complete, record the ending mass.

    13. Calculate the amount of mass changed.


Analysis:

  1. As you know, scientists write chemical reactions like mathematical formulas. The reactions are on the left of the arrow & the products are on the right of the arrow.

    Reactants → Products

    The effervescent tablet contains a chemical called sodium bicarbonate (baking soda!). This chemical reacts with water according to the following reaction:

    H20 + NaHCO3 → NaOH + CO2 + H20

    Element

    Hydrogen

    Carbon

    Oxygen

    Sodium

    Reactants

    3

    Products

    Element

    Hydrogen

    Carbon

    Oxygen

    Sodium

    Reactants

    3

    Products

    1. Count the number of each element, on each side of the equation, & record below. For example, there are 3 hydrogens on the reactant side.


    2. Is this reaction “balanced”? Explain.


  2. Compare part 1 of this reaction, the open system, to part 2, the closed system. What did you notice?