CP Chemistry
Name: ________________________________________ Date: _____________ Mods: ___________
Partners: _________________________________________________________________________
The Law of Conservation of Mass Lab
Background:
The Law of Conservation of Mass states that matter is neither created nor destroyed during a
chemical reaction and that the mass of a system should therefore remain constant during a chemical
process. This means that during any chemical reaction, the sum of the masses of the products of a
reaction must be the same as the sum of the masses of the reactants. In this experiment, you will
determine whether mass is conserved by examining a simple chemical reaction and comparing the
mass of the system before the reaction with the mass of the system after the reaction.
Pre-Lab Questions: Be sure to answer in complete sentences (when appropriate) to receive full
credit.
1. Read the procedures on the following page and highlight any crucial steps that you are worried
you may miss. You should also read the post-lab questions to familiarize yourself with what
information you will need to comment on at the conclusion of the lab.
2. What is the Law of Conservation of Mass?
3. The reaction that takes place in this lab can be summarized as follows:
NaHCO3(s)
+
Sodium bicarbonate +
CH3COOH(aq) 
Acetic acid

NaCH3COO(aq) + H2O(l) +
CO2(g)
Sodium acetate + Water + Carbon dioxide
If 5.6 g of NaHCO3 combines with 4.1 g of CH3COOH to make 2.3 g of H2O and 1.2 g of CO2, how
many grams of NaCH3COO are formed?
Safety Requirements:
Safety goggles are required for this lab. Open-toed shoes are not allowed and long hair must be tied
back.
Materials:
Weighing boat/spatula
Electronic balance
50 mL graduated cylinder
2 plastic cups
Glass stirring rod
Quart size Ziploc bag
Rubber band
Water bottle
Sodium Bicarbonate (baking soda)
5% Acetic Acid solution (vinegar)
Procedure for Part I (Open System):
1. Find the cup labeled baking soda. Place the cup on the electronic balance, zero the balance,
and then measure out 2-3 g of baking soda.
2. Using the 50 mL graduated cylinder, measure 50 mL of vinegar and transfer it to the second
plastic cup labeled vinegar.
3. Place BOTH cups (with contents) on the electronic balance and record this initial mass in
your data table on the following page.
4. Remove both cups from the balance and carefully pour the vinegar into the cup containing the
baking soda. Add only a small amount of vinegar at a time to avoid a strong reaction that
causes splattering and the loss of part of the reactants or products. Stir this mixture with the
glass stirring rod to ensure the reaction has gone to completion. Record your observations
below your data table.
5. When the reaction has finished, place BOTH cups back on the balance (one cup with
contents, the other should be empty) and determine the final mass of the system. Record the
final mass in your data table.
6. Calculate the change in mass.
7. Pour the product down the sink and thoroughly rinse and dry both cups.
Procedure for Part II (Closed System):
8. Place the now dry cup labeled baking soda on the electronic balance, zero the balance, and
then measure out 2-3 g of baking soda into the cup.
9. Pour the baking soda into one corner of the Ziploc bag.
10. Tie a rubber band around the bottom corner of the bag, which now contains the baking soda,
to keep the baking soda separate from the vinegar which will be added shortly.
11. Using the 50 mL graduated cylinder, measure 50 mL of vinegar and transfer it to the opposite
corner of the Ziploc bag. Seal the Ziploc bag.
12. Carefully carry the Ziploc bag (with contents) over to the electronic balance. DO NOT allow
the baking soda and vinegar to mix!! Make sure the entire bag is on the balance. Record this
initial mass in your data table on the following page.
13. Remove the rubber band and allow the baking soda and vinegar to mix. You may use your
fingers to help the reactants mix together. Record your observations below your data table.
14. When the reaction appears to have finished, place the still sealed Ziploc bag back on to the
balance and determine the final mass of the system (make sure you place the rubber band on
the balance at this time also so that no initial portion of the experiment is missing). Record the
final mass in your data table.
15. Calculate the change in mass.
16. Thoroughly rinse out and then dry the baking soda cup for the next class. Dump the contents
of the Ziploc bag down the drain. Turn the bag inside out and thoroughly rinse and dry the bag
for the next class.
Data Table
Initial Mass of
Reactants (g)
Open System
Closed System
(in Ziploc bag)
Observations for Open System:
Observations for Closed System – in Ziploc bag:
Final Mass of
Products (g)
Change in Mass
(g)
Post-Lab Questions: Be sure to answer in complete sentences to receive full credit.
1. Define all evidence indicating that a chemical change occurred in this experiment.
2. Was the Law of Conservation of Mass violated in Part I of the experiment (open system)?
Explain.
3. Was the Law of Conservation of Mass violated in Part II of this experiment (closed system)?
Explain.
4. Was this lab useful in proving the Law of Conservation of Mass? Explain why.
5. If the mass of your reactants was not similar to the mass of your products for the closed
system, what are two possible explanations?