Acid-Base Titration of an Eggshell

Procedure:

1.  Make data and calculations table like the following:

Titration Steps
Mass of entire eggshell
Mass of ground eggshell sample
Volume of HCl added
Volume of NaOH added
Volume of HCl reacting with NaOH
Volume of HCl reacting with eggshell
Number of moles of HCl reacting with eggshell
Number of moles of CaCO3 reacting with HCl
Mass of CaCO3
Percentage of CaCO3 in eggshell sample

2.  Remove the white and yolk from the egg. Discard in waste container. Wash the shell with with distilled water, and carefully peel all membranes from the inside of the shell. Discard the membranes. Place ALL of the shell in a pre-massed beaker, and dry the shell in the drying oven for about 15 min.

3.  Remove eggshell and beaker from the oven. Cool in a desiccator. Mass the beaker and eggshell.

4.  Place the shell in a clean mortar, and grind to a very fine powder.

5.  Transfer about 1.00g to weighing paper and mass to nearest 0.01g. Record in table. Place the eggshell sample in a clean 50mL bottle or Erlenmeyer flask.

6.  Rinse one of the buret by placing about 30mL of HCl in the buret and opening stopcock. Collect in a “waste” beaker. Repeat the same procedure using NaOH with the other buret.

7.  Fill the acid buret with the acid and add 50.0mL of HCl to the bottle (or flask) with the eggshell, and record precise volume in table. Swirl gently for about 3-4 minutes. Rinse the sides of the flask with about 10mL of distilled water. Add 3 drops of phenolphthalein solution.

8.  Fill the base buret with NaOH. Titrate this solution from step 7 with the NaOH. Be sure to add the NaOH slowly, and swirl until a faint pink color persists, even after it is swirled. Also be sure Athe base drops end up in the reaction mixture and not on the side of the bottle (or flask). Record volume of base used in table.

9.  Clean all apparatus and your lab station. Dispose of chemical and solutions as directed by the teacher. Wash your hands thoroughly before you leave the lab and after all work is finished.

Analysis:

1.  The calcium carbonate in the eggshell undergoes a double-replacement reaction in step 7. Then the carbonic acid formed decomposes. Write a balanced chemical reaction for these reactions.

2.  Write a balanced chemical equation for the acid-base neutralization of the excess unreacted HCl with the NaOH.

3.  Using the relationship between molarity and volume of acid and molarity and volume of base needed to neutralize the acid, calculate the volume of HCl solution that was neutralized by the NaOH. Then subtract this amoun t from the initial volume of HCl to determine the volume of HCl reacted with the CaCO3 in the eggshell.

Conclusions:

1.  Use the stoichiometry of the reaction in Analysis item 1 to calculate the numver of moles of CaCO3 that reacted with the HCl, and record this number in your table.

2.  Workers in a lab in another city have also tested eggs, and they found that a normal eggshell is about 97% CaCO3. Calculate the percent error for your measurement.

3.  What possible sources of error can you identify in this procedure? What can be done to improve the procedure?