# Lonestar College Module 5 Law of Conservation of Mass Questions

Module 5; Law of Conservation of MassIntroduction and Pre-Laboratory Exercise
Physical changes include mixing, separating and changes of state. Chemical changes involve breaking bonds and forming
new bonds to create new chemicals. In either case, the changes follow a fundamental law of nature: you can change
matter, but you cannot create or destroy it. This law is called the Law of Conservation of Mass. If you melt an 18 g ice
block, you should end up with 18 g of water. If you add 1 g of sugar to your 50 g of iced tea, you should end up with 51 g
of sweet iced tea. If you run a current through 10 g of water you should end up with 10 g of H2 and O2 gases.
Answer the following questions by applying the Law of Conservation of Matter. Show your work.
1.
Determine whether each of the following statements is consistent with or inconsistent with the laws of conservation
of mass.
(a) 127.10 g of Cu reacts with 34.00 g of O2 to produce 159.10 g of CuO.
(b) 76.15 g CS2 reacts with 96.00 g of O2 to produce 44.01 g CO2 and 128.14 g of SO2.
2.
Determine the value of X in the following otherwise correct descriptions of a chemical reaction.
(137.32 g PCl3) + (X g H2)
(34.00 g PH3) + 109.38 g HCl).
3.
A 4.2 g sample of sodium hydrogen carbonate is added to a solution of acetic acid weighing 10.0g. The two
substances react releasing carbon dioxide gas to the atmosphere. After the reaction, the contents of the reaction
vessel weigh 12.0 g. What is the mass of carbon dioxide given off during the reaction?
4.
A 1.00 g sample of solid calcium carbonate is added to a reaction flask containing 1000 g of hydrochloric acid
solution. The calcium carbonate slowly dissolves in the acid solution as evidenced by the generation of carbon
dioxide gas. After 5 minutes of reaction, 0.21 g of carbon dioxide gas has been given off. At that time, what is the
mass in grams, of the reaction flask contents?
Note: In part 2 of this lab, you will be doing a “kitchen” experiment. You will need the following
materials for your experiment: three balloons, three empty water bottles, vinegar, baking soda
1
Law of Conservation of Mass
Data sheets and Post Lab Questions
Part A: Watch the following lab demonstration and complete the data table below:

Complete the data table for Part A
Before Mixing
Mass altogether of Erlenmeyer flask, vinegar, balloon and baking soda: _____________
Observations of balloon, soda and vinegar before mixing:
After Mixing
Mass altogether of Erlenmeyer flask, vinegar, balloon and baking soda: _____________
Observations of reaction and products:
Post Lab Questions:
1) Was the change in the demonstration a physical or chemical change?
2) In this experiment gas was produced. Gas has mass. Why didn’t the mass of the system
therefore increase during this reaction? (Use the term “law of conservation of mass” in
3) The gas that was created is CO2. Where did the carbon and the oxygen come from that
2
Law of Conservation of Mass
Data sheets and Post Lab Questions
Part B: You will do this experiment in your kitchen. You need baking soda, vinegar, three
balloons and a three empty water bottles.
Repeat the experiment you saw in the demonstration with baking soda in the balloon and
vinegar in the water bottle. Use measuring cups for the vinegar and teaspoons for the baking
soda.
TRIAL #1
How much vinegar will you use? ______________
How much baking soda will you use? ______________
TRIAL #2
For your second trial, modify your amounts so that you get more CO2 produced (a bigger
balloon).
How much vinegar will you use? ______________
How much baking soda will you use? ______________
TRIAL #3
If that didn’t work, try something else. If it did work, try increasing the amount of CO2 a different
way.
How much vinegar will you use? ______________
How much baking soda will you use? ______________
Take a picture of your three balloons and attach them to the lab.
Post Lab Questions:
1. Are your results consistent with the Law of Conservation of Mass? Explain.
2. If you were going to do a few more trials, what would you test next?
3

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