How much calcium carbonate is in an eggshell

Eggshell was ground, and a mass of 0.1 gram was transferred to an Erlenmeyer flask. 150 drops of acid were added to the eggshell and agitated for about three minutes. The sides of the flask were washed down, then phenolphthalein indicator was added to the solution.

Base was added from the base solution until a color change was noted in the indicator. The amount of base added was recorded when the color change was permanent.

The volume of base added was used to calculate the amount of acid that had not reacted with the eggshell. The amount of acid that had reacted with the eggshell was calculated by subtraction. This amount of acid was used to calculate the amount of calcium carbonate present in the eggshell, and then the percent of calcium carbonate present in the eggshell.

Observations:

Calibration of the pipettes was done to determine the volume of liquid being produced by each pipette as they were used in the reaction.

Acid pipette Base Pipette

Trial

Initial Acid

Final Acid

Initial Base

Final Base

5.0 ml

5.8 ml

5.0 ml

5.8 ml

2

5.8 ml

6.7 ml

5.8 ml

6.6 ml

3

6.7 ml

7. 6 ml

6.6 ml

7.5 ml

Acid pipette: Average volume 0.87 ml/20 drops: 0.043 ml/drop

Base pipette: Average volume 0.83 ml/20 drops: 0.042 ml/drop

The mass of the eggshell and volumes of HCl and NaOH used were recorded.

Mass of ground eggshell: 0.12 g

Number of drops HCl added: 150

Volume of HCl added: 6.45 ml

Number of drops NaOH added: 97

Volume of NaOH added: 4.07 ml

When the HCl was added to the eggshell, it seemed to bubble. This is no doubt the result of the formation of CO2 gas. Once this reaction was complete, NaOH was added to the solution to neutralize the acid and to enable calculation of the amount of acid used in the reaction. Phenolphthalein was used as an indicator to determine when the acid/base reaction was complete. 97 drops of base were added to neutralize the acid, then the volume of acid and base used was calculated using the calibration values for each pipette.

The amount of acid that reacted with the eggshell can be calculated by subtraction. 6.45 ml-4.07 ml=2.38 ml acid. 2.38 ml of 1.0 M HCl contains 2.38 x 10-3 moles HCl.

Using the equation: 2HCl + CaCO3 --> CO2 + H2O + CaCl2, the amount of CaCO3 that will react with 2.38 x 10-3 moles of HCl can be determined.

2.38 x 10-3 mole HCl x 1 mole CaCO3 = 1.19 x 10-3 mole CaCO3

2 mole HCl

The mass of 0.0012 moles of CaCO3 can be calculated using the equation:

0.0012 moles CaCO3 x 100.088 g = 0.12 g CaCO3

1 mole

Thus, 0.12 g of CaCO3 were contained in the 0.12 g eggshell sample, or the eggshell was 100% composed of CaCO3.

The established value for the composition of eggshell is 97%. Our error in this lab was:

97% - 100% = 3.1% error

97%

Conclusions: The calculated percent CaCO3 in the eggshell was 100%. This differs from the established value of 97% by 3.1%. Possible errors in the determination of calcium carbonate might include difficulty in determining the endpoint in the acid/base reaction, improper indicator used (it was later discovered that thymolphthalein had been used instead of phenolphthalein), and human error in measurement. It is, however, obvious that most of the eggshell is composed of CaCO3, and this quantity can be determined by titration with known concentration acid and base solutions.