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Acid-Base Mole Ratio (0 Favorites)

LAB in Limiting Reactant, Titrations. Last updated April 26, 2019.


Summary

In this lab, students study several concepts, including acid-base reactions, limiting reactants, and stoichiometry, by observing the contained reaction of acetic acid (diluted vinegar) with sodium hydrogen carbonate (baking soda) in an unconventional, cost effective titration.

Grade Level

High school

AP Chemistry Curriculum Framework

  • Big Idea 3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons.
    • 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
    • 3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results.
    • 3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.13 The student can interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base.

Objectives

By the end of this lab, students should be able to

  • Find the stoichiometric ratio of acetic acid when reacted with sodium hydrogen carbonate to form carbonic acid and sodium acetate.
  • Determine which reactant is the limiting reactant.
  • Observe how the amount of gaseous product is affected by the limiting reactant.

Chemistry Topics

  • Acid-base reactions
  • Titrations
  • Stoichiometry
  • Limiting reactants
  • Chemical reactions
  • Gas Laws

Time

Teacher Preparation: 30 minutes

Lesson: 45 minute class period

Materials

  • Vernier Logger Pro
  • Computer
  • Vernier Gas Pressure Sensor
  • 1000 mL flask
  • Stopper
  • Tapered valve connector
  • Plastic tubing
  • 100 mL graduated cylinder
  • 12 mL syringe
  • Small beaker (exact size does not matter, just something to hold the vinegar)
  • 2.5 % acetic acid (store bought vinegar diluted)
  • 0.03 M sodium bicarbonate solution (baking soda water mixture)
  • Bottle of distilled water (for rinsing)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Students should wear a lab apron and closed toes shoes at all times.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Tape any glass flasks used to produce gases to guard against possible shattering.
  • Press down on the stopper at all times, to ensure that the stopper does not pop off.
  • Use caution when removing the stopper after measuring the pressure, as the suction in the flask may require greater force to remove.
  • Keep the MSDS reference documents on hand for all chemicals.

Teacher Notes

  • This laboratory experiment accompanies a Chemistry Solutions article in the November 2015 issue.
  • This lab is a great substitute to the stereotypical balloon lab done by those who care not about results but about lab experience. Though balloons are fun, it is important for students to see how the reaction actually produces scientifically quantifiable results that can be applied to laws and formulas. The results are much more precise than those obtained from balloons, since there is much less room for error.
  • Since this lab ties together several concepts, it is recommended this be done near the end of first year or near the beginning of second year.
  • The following preparations should be made by the teacher before the day begins:
    • Gather all the glassware needed - flask, graduated cylinder, and beaker. A 12 mL syringe is also needed to measure out the vinegar solution. Everything will need to be rinsed between trials with water, so it is important to have this on hand at every table.
    • You will need plenty of .03 M sodium bicarbonate solution. To make this solution you need 2.52 grams of baking soda for every liter of water. Recommendation is for one gallon of solution per two student groups.
    • The acid reagent is 5% store-brand vinegar that has been diluted to half concentration (one part vinegar, one part water). Not as much of this is needed, so make it in small batches just before the lab. The whole investigation can be done in 45 minutes, but when experimenting over the course of more than one day it is best to remake the solution for the second set of trials.
    • The reagent solutions can be easily and safely stored, even in one-gallon plastic jars, allowing teachers to prepare solutions days in advance.
  • (Optional) AP/Honors Inquiry: Students can use the vinegar and the pressure probes to determine the concentration of the baking soda from the measurement of the pressure. From this, the students can devise their own procedures and use it as a template for labs with other molarities of baking soda. In this case, honors teachers should carefully review the procedures to make certain that the partial pressures do not increase to a hazardous level.

Associated Content:

  • Reaction explanation video (below). This video describes the molecular-ionic level of a reaction between the two chemicals used in this lab. An quantitative explanation provides meaning for limiting reactant and excess reactant, as well as why the graph increases linearly but then flattens out.

For the Student

Lesson

Background

Stoichiometry, a branch of chemistry dealing with ratios of chemical molecules, is a commonly utilized tactic to prevent the waste of resources. Furthermore, this method allows efficient reactions to take place. We will explore this concept in this lab.

Objectives

  • Find the stoichiometric ratio of acetic acid when reacted with sodium hydrogen carbonate to form carbonic acid and sodium acetate.
  • Determine which reactant is the limiting reactant in the two phases of the investigation.
  • Observe how the amount of gaseous product is affected by the amount of limiting reactant.

Materials

  • Vernier Logger Pro
  • Computer
  • Vernier Gas Pressure Sensor
  • 1000 mL flask
  • stopper
  • tapered valve connector
  • plastic tubing
  • 100 mL graduated cylinder
  • 12 mL syringe
  • beaker
  • 2.5 % acetic acid (vinegar)
  • 0.03 M sodium bicarbonate solution (baking soda water)
  • bottle of distilled water (for rinsing)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wear a lab apron and closed toes shoes at all times.
  • Wash your hands thoroughly before leaving the lab.
  • Follow your teacher’s instructions for clean-up and disposal of chemicals.
  • Press down on the stopper at all times, to ensure that the stopper does not pop off.
  • Use caution when removing the stopper after measuring the pressure, as the suction in the flask may require greater force to remove.

Procedure

  1. Obtain and wear safety goggles and lab apron.
  2. Open Logger Pro on your computer and plug in the Gas Pressure Sensor, and then connect it via plastic tubing to a rubber stopper that fits the flask.
  3. Measure 50 mL of the sodium bicarbonate solution using the graduated cylinder and add it to the flask. Calculate the number of moles at this volume.
  4. Plug the flask with the stopper connected to the gas pressure sensor and record the final pressure after 180 seconds. This will be your control pressure value which you can use for partial pressure calculations.
  5. Empty and rinse the flask thoroughly with distilled water.
  6. Measure 50 mL of the sodium bicarbonate solution using the graduated cylinder and add it to the flask.
  7. Use a 12 mL syringe to measure 1mL of the vinegar from the beaker.
  8. Begin data collection and quickly add the vinegar to the flask, then immediately plug the flask with the stopper.
  9. Swirl the flask while collecting data for 180 seconds (3 minutes). Stop collection and store the data. Record the final pressure in the data table.
  10. Empty and rinse the flask thoroughly with distilled water.
  11. Repeat steps 6-10, increasing the amount of diluted vinegar added to the flask by 1 mL in each trial until you have recorded trials through 7 mL of vinegar added.

Vinegar added (mL)

Pressure (kPa)

0
1
2
3
4
5
6
7
Data

Analysis

  1. By looking at the balanced chemical equation, what is the expected equivalence point for the baking soda solution and diluted vinegar solution?
  2. Graph the data collected from the experiment. Analyzing the graph that your data produced, what is the equivalence point in relation to the data you collected?
  3. What is the percent error of your experiment? (use the theoretical equivalence point as your expected result)
  4. What may have caused the percent error that you found?

Conclusion

  1. Were there any noticeable discrepancies in your data, and if so, what could have caused them?
  2. What could have contributed to sources of non-human error? (i.e., not anything that you could have done, like stirring too vigorously)
  3. In doing this experiment, how can you relate acid-base reactions, stoichiometry, and limiting reactants?