Conservation of mass

Question:

Are the masses of baking soda and vinegar conserved when I mix them together in an open system?

Background:

The total mass of substances that undergo a chemical reaction (reactants) in a chemical reaction must equal the total mass of substances created by the chemical reaction (products). This is called the Law of Conservation of Mass.

Early scientists often did not consider that the chemical reactions they were investigating might generate gaseous products. They did not always anticipate that gases in the atmosphere would participate in their experiments.

We will be doing an experiment that might have given confusing results for early chemists or alchemists. We will mix baking soda and vinegar in an open beaker. This open system experiment will allow any gases evolved to escape.

Hypothesis:

When I mix baking soda and vinegar together in an open system, the mass of the reactants (baking soda and vinegar) will equal the mass of the products.

Equipment:

Procedure:

  1. Record the mass of a empty 150 ml. beaker. Place 4 to 5 grams of baking soda into the empty beaker. Record the mass of the baking powder + beaker. Calculate the mass of the baking soda.
  2. Record the mass of the empty 100 ml. graduated cylinder. Measure about 100 ml. of vinegar in the graduate. Record the mass of the vinegar + graduate. Calculate the mass of the vinegar.
  3. Carefully pour the vinegar, a little at a time into the beaker containing the baking soda. Pour gently along the inside wall of the beaker so splashing is minimized. Gently swirl the beaker to make sure the reactants are well mixed. Record any observations in the space under the data table.
  4. When the reaction is finished, weigh the beaker to determine the final mass of the system (baking soda and vinegar reaction products, and beaker). Calculate the mass of just the reaction products.

Data:

  1. ______________ Measured mass of 150 ml. beaker with baking soda.
  2. ______________ Measured mass of empty 150 ml. beaker.
  3. ______________ Calculated mass of baking soda (difference of numbers 1 and 2).
  4. ______________ Measured mass of 100 ml. graduated cylinder with vinegar.
  5. ______________ Measured mass of 100 ml. graduated cylinder.
  6. ______________ Calculated mass of vinegar (difference of numbers 4 and 5).
  7. ______________ Calculated mass of reactants (sum of numbers 3 and 6).
  8. ______________ Measured mass of 150 ml. beaker + reaction products.
  9. ______________ Measured mass of empty 150 ml. beaker (from number 2 above).
  10. ______________ Calculated mass of reaction products (difference of numbers 8 and 9).

Observations:

The reaction proceeded with vigorous bubbling and frothing.

Error calculation:

((massproducts - massreactants) / massreactants) * 100 = % error
 
  ((______ g - _____ g) / ______ g) * 100 = _______ % error

Conclusion:

I attempted a simple experiment to demonstrate the law of conservation of mass. I hypothesized that following an uncontained baking soda and vinegar reaction (an open system), the mass of the reactants would equal the mass of the products. The data did not support this hypothesis. The mass of the reactants was _____ grams, however the product mass was ______ grams. This was a - ____% error. Aside from the typical errors in measurement that might occur, I believe the error was due to a gaseous product that I did not capture with this experimental setup. During the reaction, I observed a frothy bubbling. This vigorous bubbling decreased during the two or three minutes following the initial reactant mixing. Safely containing all the reaction products should result in the reactant mass equaling the product mass. A sealable plastic bag might be suitable for creating such a closed system if the volume of the gases was small.