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Conservation of Mass Lab
Chemistry Materials from Ray Tedder

Exploring the Law of Conservation of Mass

 The Law of Conservation of Mass:
In a chemical reaction, the mass of the products equals the mass of the reactants.
This lab activity will take several days to a week to complete.
The 2 labs follow a classroom activity in which students are assigned to research the life of Antione Lavoisier.  From that research, students are to cull a series of unique facts about Lavoisier or about his life and present one of those facts to the class.  No 2 students can give the same fact, and the order in which students will be called will not be given in advance, so each student should have a list of at least 30 facts.  You may simply print web pages, the underline and number your facts.  But each student must be familiar enough with their list of facts that they can discuss them without reading the printouts when call upon.
Following these 2 labs, students will explain the underlying theory behind the Law of Conservation of Mass-Energy using an analogy taken from their life experience. 
Lab Measurements

It is particularly important to make the best possible measurements for this lab.  A failure to follow measuring procedures learned in earlier labs will result in a failure to identify the phenomenon for which we are looking.  If you cannot find the phenomenon, then you cannot make a reasonable hypothesis for the phenomenon, you cannot determine a method for testing your hypothesis, and you cannot determine of your hypothesis was correct or not.  Your lab grade is dependent on successfully completing all these steps, and making correct measurements is essential.  If you cannot determine and identify the phenomenon during the allotted time, then you will have to come in before or after school if you wish to complete the process.  So, following good lab procedure is essential. 


Lab Procedures

Mark a small Erlenmeyer flask to be sure that you will be using the same equipment in the follow-up lab.  Determine the mass of this small Erlenmeyer flask.  Also, determine the mass of an amount of baking soda, and the mass of an amount of a strong acid of known molarity. 


Wearing gloves, carefully pour the acid into the Erlenmeyer flask.  Pour the acid from lip of the flask down the inside edge of the flask and into the baking soda so that no acid is splashed.


Observe and record the results. 


Cautiously swirl the acid in the Erlenmeyer flask until the baking soda is completely dissolved.  Observe and record the results. 


Determine the masses of all the materials again.  Observe and record the results. 


Compare the masses of your reactants and your products.  Observe and record the results. 


As a group determine what happened that is unexplained and hypothesize as to what caused this.  Record this hypothesis.


Post Lab Procedures and Pre-lab for Follow-up Lab

Once you have determined a hypothesis for the phenomenon, you and your lab group should determine a lab procedure for testing it.  Once you have agreed upon a defendable procedure, determine what additional equipment or materials will be needed and submit that to the instructor for an assessment for his/her review to insure that your procedure is reasonable and possible.  You must submit your list of needed materials in time for them to be obtained from an outside source if necessary.  You should also submit your list of needed materials in time for a thorough review so that changes can be made if necessary.  Be prepared to perform your follow-up experiment to test your hypothesis on the assigned day.   


As part of your post lab, your lab group should determine a reasonable balanced equation for this reaction between the acid and sodium bicarbonate (NaHCO3).  You can expect the bicarbonate ion (HCO3-) to remain intact from reactants to products.  This will be needed for your analysis.

Sample Lab Report:

Note that this is a sample of a lab report written by a student.  It is not perfect and no attempt has been made to correct the errors in it.  The data (measurements and observations) found in this report will not be the same as that found in your lab report.  Read the web page on plagiarism before writing your lab report and follow the "Formal Lab Report Guide."


----------------------------------- Page 1 -----------------------------------  


















Determining what happens to sodium bicarbonate and acetic acid when they are combined in a solution, and why solution loses its mass.














Writer:  --------, ---------

Partners:  --------, ---------

                --------, ---------

                --------, ---------

28, Oct, 03 and 31, Oct, 03

Room 215A, Dorman High School, Roebuck SC   29376

-------------------------------- Pages 2, 3, 4 --------------------------------

V. Results


In the lab the results of what happened where pretty straightforward as far as observation went.  On the 28th of October the first part of the lab was completed, which was to observe what happens when a mixture of sodium bicarbonate (baking soda) and acetic acid come together to make a solution.  The observations were clear, when the sodium bicarbonate was added to the acetic acid bubbles automatically formed and started to increase rapidly, also a strong scent was given off mainly because of the acetic acid.  In some parts of the lab some people saw fumes rising off the top of the Erlenmeyer flask that was used, and others did not.  This could be due to the fact that people were seeing the reaction take place at different angles during the lab.  When the solution had stopped fizzing, and for the most part calmed down, it took on a gelatin like form and continued to harden.  At this point the solution was measured, and to the confusion of the lab groups, the mass never stayed the same.  It was continually dropping, here are some measurements for example:




It is apparent that the mass continued to drop rapidly during the measurement of the solution.  The solution also hardened to almost a rock type solid.  The speculation as to why the mass is that the solution was turning into as gas and releasing into the atmosphere of the room.  In the lab on the 31st of October the lab groups were expected to have final procedures to find out exactly what was happening to the solution as its mass rapidly dropped.  All the procedures that were to be done were repeated first measuring all the materials, the Erlenmeyer flask weighed 119.55g, the beaker weighed 48.25g, the acetic acid 20.05g (at 25mL), and the baking soda weighed 35.42g.  After all that was weighed the acetic acid and the baking soda were put together, but a balloon was used to put the baking soda in so the gas would be trapped in the balloon (the balloon weighed 3.012g, and w/ the baking soda it weighed 38.01g) and the measure of the reactants together was 175.0 g.  As was expected, the gas was trapped in the balloon, making the balloon expand. The final products were almost the same as the reactants they weighed 174.58 g.  When finding the percent error both of these numbers were used first subtracting the accepted mass, which was 175.0, and the experimental mass, which was 174.58 g, and then dividing the final number by the accepted mass (see appendix a).

VII. Discussion Analysis   (It should be noted that this lab used only half the ingredients needed in this lab due to loss of products in experiment before hand).

The lab accomplished mostly to show how the states of matter change and how mass is lost during a combination of two compounds.  When the first lab was finished on the 28th of October, only one conclusion was made, that mass was being was being lost in the reaction w/ the sodium bicarbonate, and the acetic acid.  To prove this all that was needed was a simple balloon to trap the gas and prove that mass being lost, by expanding the balloon.  When the lab was finished the observations were even with the results and the finished products were as expected.  It was proven that mass was lost in the form of gas releasing from the Erlenmeyer flask. 

Balanced Equation:


Also because the air around the experiment and in the atmosphere, it makes the weight of the C02 (see balanced equation) balance out because the pressure of both of the elements are the same.  The only way error could come about is if some of the gas was release into the classroom and not directly into the balloon causing the measurement to be unequal, and lost product, which could also be a factor if acetic acid and baking soda was lost in transferring into the Erlenmeyer flask. There was in fact mass lost (mass really wasnt lost according to law of conservation of mass, but the mass of the gas escaped and some product was lost making the mass different), in this lab causing a percent error, which was 0.24% the equation is:

accepted mass-experimental mass/accepted mass=%error or 175.0-174.58/175.0= 0.25% error.  The math for proving this, and the math for finding the molarity of the acetic acid, is attached in an appendix labeled A. Also this lab showed and proved the law of conservation of mass that no mass can be created or destroyed during a chemical reaction, and no mass was created or destroyed, even though the mass separated into different forms such as a gas, and a solid, it was all still there in some form, just not as it was originally.

If you have questions about this page, please contact Ray Tedder at: