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Conservation of Mass

Demonstrating conservation of mass through 2 chemistry lab experiments

Procedure 1

Empty test tube- 18.200g
Closed test tube- 19.128g
Vinegar (5ml)- 23.213g
NaOH (5ml)- 23.283g
Closed tube of NaOH (5ml)- 24.211g

1. Record the following masses in grams:

(a) an open, empty test tube:

(b) a closed, empty test tube:

(c) the test tube containing 5 mL vinegar:

(d) the test tube containing 5 mL sodium hydroxide:

(e) the closed test tube after the reaction:

2. Calculate and record the combined mass of the reactants vinegar and sodium hydroxide (in grams).

3. Calculate and record the mass of the contents of the test tube after the reaction (in grams).

4. Compare the two masses before and after the reaction. Calculate the percent deviation between the original mass and the measured mass after the reaction, which is defined as:

percent deviation = |(mass before reaction) - (mass after reaction)| / (mass after reaction * 100%)

According to the law of conservation of mass, we expect the percent deviation to be zero, but experimental measurements rarely give perfect results. The percent deviation is an indication of how well the experiment conforms to our expectations.

5. Explain what may have caused any discrepancy between the two values of the masses and whether you have confirmed the law of conservation of mass.

Procedure 2
Empty flask- 91.780g
Closed empty flask- 96.548g
Flask w/ Alka seltzer powder(1g)- 97.548g
Empty flask2- 91.780g
Flask2 H2O(40ml)- 131.780g
Flask2 H2O(40ml)+Alka seltzer powder(1g)- 137.548g
Assignment 1 of Procedure 2

1. Record the following masses, making sure to use the correct number of significant digits:

(a) mass of an open, empty flask (g)

(b) mass of a closed, empty flask (g)

(c) mass of a closed flask plus 1g Alka Seltzer powder (g)

(d) mass of an open flask with 40 mL water (g)

(e) mass of the closed flask with 1g Alka Seltzer and 40 mL water, after the reaction (g)

2. Calculate the combined mass of the reactants, Alka Seltzer and water (g)

3. Calculate the mass of the contents of the flask after the reaction.

4. Compare the two masses, before and after the reaction. Calculate the % deviation as:

% deviation = |(mass before reaction) - (mass after reaction)| / (mass after reaction) * 100%

5. Explain what may have caused any discrepancy between the 2 masses, and whether you have proved the law of mass conservation.

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Solution Preview

Both experiments are very similar to each other, so I will explain the general concepts followed by a discussion for each question as it applies to both experiments.

The law of conservation of mass states that the amount of matter going into a reaction (of which matter has mass) should be exactly the same as the amount coming out of a reaction. By extension, so should the masses. To illustrate, imagine taking two Lego structures and deconstructing both of them and using all the building materials to build one new structure. Since you're using all the blocks from the original structures, your new structure should be the same mass as the sum of both the original structures. The same applies, but with atoms and molecules undergoing a chemical reaction.

Both your experiments put this fundamental law to the test. In the first case, you're working with a simple neutralization reaction between an acid and a base. Your acid is vinegar, your base is the sodium hydroxide.

CH3COOH (acetic acid in vinegar) + NaOH <--> H2O + NaCH3COO

In the second experiment, water becomes a solvent for the components of the Alka seltzer to react in. For the purposes of this question, the equation of the reaction isn't necessarily important, but for curiosity's sake:

C6H8O7 (citric acid) + 3NaHCO3 (sodium bicarbonate) ? 3H2O + 3CO2 + Na3C6H5O7

What's ...

Solution Summary

The following posting discusses problems that involve the law of conservation of mass.

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