CHEM 1102

Determination of Calcium Carbonate in Eggshells

Purpose

To measure the percent by mass of CaCO₃ in an eggshell.

Introduction

The major component of eggshells is calcium carbonate (CaCO₃).
This analysis will be done volumetrically by using a characteristic reaction of carbonate compounds, namely their reaction with acids. Calcium carbonate (limestone) is very insoluble in pure water but will readily dissolve in acid according to the reaction

      2HCl (aq) + CaCO₃(s) ----> Ca²⁺ (aq) + CO₂(g) + H₂O + 2Cl^- (aq)

      This reaction cannot be used directly to titrate the CaCO₃ because it is very slow when the reaction is close to the endpoint. Instead the determination is achieved by adding an excess of acid to dissolve all of the CaCO₃ and then titrating the remaining H₃O⁺ with NaOH solution to determine the amount of acid which has not reacted with the calcium carbonate. The difference between amount of the acid (HCl) initially added and the amount left over after the reaction is equal to the amount used by the CaCO₃. The reaction used to determine the leftover acid is

      HCl (aq) + NaOH (aq) ----> H₂O + Na⁺(aq) + Cl^- (aq)

Stockroom: Things to borrow and return on the same day.

• Buret and brush (~$75)

Procedure

1. Each group of two students should obtain one egg and the necessary glassware from the stockroom.
2. Break the egg into a beaker. Add water to the egg and stir before pouring down the drain.
3. Wash the shell with deionized water and peel off all of the membranes from the inside of the shell. Dry it with a paper towel and put into a labeled beaker.
4. Dry the shell for about 10 minutes in the oven.
5. Grind the shell to a fine powder in a mortar.

• The rest of this experiment is done individually.

1. Accurately weigh between 0.450 and 0.550 g of dried shell into each of 3 labeled 125 ml conical flasks. Be certain you record the mass of shell for each flask in your notebook.
2. Add several drops of ethanol to each flask. This acts as a wetting agent and helps the HCl dissolve the CaC0₃.
3. Slowly pipet 10.00 ml of 1.0 M HCl solution into each flask. Swirl the flask to wet all of the solid. Any excess HCl should be disposed of in the sink by diluting with water.
4. Heat the solutions in the flasks until they begin to boil and allow to cool. Rinse the walls of the flask with water from your wash bottle.
5. Add 3-4 drops of phenolphthalein (a.k.a PhPL) to each flask.
6. Using a funnel, partly fill a clean buret with 0.100 M NaOH solution to rinse it. Empty the buret into the sink. Fill the buret with the NaOH solution. Run some solution out to remove all bubbles from the tip. Replenish the solution in the buret if necessary. Read and record the initial volume to +/- 0.01 ml.
7. Titrate one sample to the first persistent pink color. When you are close to the endpoint the color will fade slowly. Add the remaining NaOH dropwise until the color remains for at least 30 sec. Read and record the final volume to + 0.01 ml.
8. Repeat the titration for the other two samples.
9. Calculate the percent calcium carbonate in each sample and the mean value. Calculate the average deviation from the mean.
10. Wash the egg residue out of the conical flask with hot soapy water and a test tube brush.

Calculations

1. Calculate the number of moles of HCl added to each shell sample. This is given by the expression:

moles HCl = (0.01000 l HCl)*(1.00 moles HCl/liter) = 1.00x10^-2 moles HCl
.

2. Calculate the moles of HCl left in each sample after the reaction with CaC0₃.

moles HCl left = (vol. of NaOH in liters)*(concentration of NaOH in moles/liter)*(1 mole HCl/1 mole NaOH)
.

3. For each sample determine the number of moles of HCl that has reacted with CaCO₃ by taking the difference between the moles of HCl added and the moles of HCl remaining after the reaction is complete.

.

4. The moles of CaCO₃ in each sample is calculated by:

moles CaCO₃ = (moles HCl)*(1 mole CaCO₃ / 2 moles HCl)
.

5. Calculate the percent CaCO₃ in each sample by using

% CaCO₃ = [(moles CaCO₃) *(100.09 g CaCO₃/mole CaCO₃)*(100)] / grams of sample
.

6. Calculate the mean value and the average deviation from the mean.

Sample Calculation

Problem:

In an experiment, 0.500 g of eggshell is dissolved in 10.00 mL of 1.00 M HCl. The volume of 0.100 M NaOH required to neutralize the leftover HCl is 29.70 mL. What is the percent CaCO₃ in the eggshell?

 Solution:

Initial moles HCl = V_HCl M_HCl

.
= 0.0100 L x 1.00 mole/L = 1.00x10^-2 mol
.

moles HCl left = V_NaOH M_NaOH

.
= 0.02970 L x 0.100 mole/L = 2.97x10^-3 mol
.

moles CaCO₃ = moles HCl reacted x (1 mol CaCO₃ / 2 mol HCl )

.

moles HCl reacted = 0.0100 - 0.00297 = 0.0070 moles

.

moles CaCO₃ = 0.0070 mols HCl x (1 mol CaCO₃ / 2 mol HCl ) = 0.0035 mols

% CaCO₃ = ( 0.0035 mols x 100.1 g/mol ) / 0.500 g = 70.0%  (report your answer to three significant figures.)

 

Conclusion:

Using complete sentences, in paragraph form, restate the average percent calcium carbonate in the eggshell and the average deviation.  What sources of error occurred in this lab?  Why did you put the eggshells in the oven?  Are eggshells  a good source of calcium?  Why would you want to avoid eating raw eggshells?