Stoichiometry of a Chemical Reaction

The following balanced equation shows the reaction between the

compounds sodium hydrogen carbonate and hydrochloric acid.

NaHCO₃(s) + HCl(aq) à NaCl(aq) + CO₂(g) + H₂O(l)

The equation states that one mole of sodium hydrogen carbonate reacts with one mole of hydrochloric acid to produce one mole of sodium chloride, one mole of carbon dioxide, and one mole of water.

In this experiment, you will react samples of three known carbonates and an unknown carbonate with hydrochloric acid. You will determine the stoichiometric relationships among the reactants and products. You will then determine the identity of the unknown carbonate by comparing its experimental results with those of the known carbonate.

OBJECTIVES

• React known amounts of carbonates and hydrogen carbonates with acid.

• Determine the stoichiometric relationships among the reactants and products.

• Identify an unknown carbonate by the amount of product formed.

MATERIALS

24-well microplate

thin-stem pipet

balance

distilled water

paper towels

PROCEDURE

Part 1: Determining Stoichiometric Relationships in a Reaction

1. Obtain a sample of a carbonate, approximately 1.0 g mass. Record the molecular formula that appears on the label of the container. Record all data for this carbonate in the Data and Observations

section.

2. Place the microplate on a balance and record the mass to the nearest 0.01 g.

3. Put all the carbonate into well A4 of the microplate. Measure and record the mass of the microplate and the sample.

4. Fill a thin-stem pipet with 8M HCl solution.

CAUTION: Be careful with HCl. It burns skin and clothing. Do not inhale the vapors. If spillage occurs, notify your teacher promptly.

5. Wipe the outside of the pipet filled with HCl, and stand it stem up in well A3.

6. Measure and record the total mass of the unreacted setup (the microplate, the sample, and the pipet with HCl solution).

7. Take the pipet from well A3 and, drop by drop, add the HCl solution to the sample of carbonate in well A4. Allow the bubbles of gas to escape after each drop before adding more HCl.

8. Continue to add acid solution one drop at a time until the carbonate has dissolved and the solution produces no more bubbles.

9. Return the pipet, stem upward, to well A3 and again mass the tray, the solution, and the thin-stem pipet. Record the mass of the complete reacted setup.

10. Dispose of the reacted chemicals as directed by your teacher.

11. Rinse the microplate with water and dry with a paper towel.

12. Repeat steps 1–11 with the other known carbonates.

Part 2: Identifying an Unknown Carbonate by Stoichiometric Principles

Repeat steps 1–11 with the unknown carbonate.

Record your data for the unknown carbonate.

DATA AND OBSERVATIONS

Part 1: Data for known carbonates

Part 2: Data for unknown carbonate

ANALYSIS

1. Write a balanced equation for the reaction of each known carbonate with HCl.

2. For each known carbonate, calculate the mass of the sample and the number of moles used. Record these values.

3. Calculate and record the mass of the unknown carbonate.

4. The difference between the masses of the unreacted and reacted setups is, in each case, equal to the mass of carbon dioxide given off. For each known and unknown carbonate, determine the mass of CO₂ lost. Record the masses.

5. Determine and record the number of moles of CO₂ produced from each reaction of a known carbonate.

6. Determine the mass of CO₂ produced as a percentage of the mass of each carbonate. Record the percentages.

CONCLUSIONS

1. From the stoichiometric relationships in the reactions in Part 1, determine the number of moles of CO₂ that theoretically would be produced by each reaction.

2. How did the actual number of moles of CO₂ produced by each reaction with a known carbonate compare with the theoretical

number of moles?

3. Because a substance reacts stoichiometrically, the outcome of a reaction can be used to identify an unknown compound. Compare the percentage of CO₂ in the unknown carbonate with the percentages of CO₂ in the three known carbonates. What is the unknown carbonate?

EXTENSION AND APPLICATION

Geologists are often interested in the percentage of carbonates in certain rocks and minerals. Find out why.