Determination of Calcium Carbonate in Eggshells
To measure the percent by mass
of CaCO3 in an eggshell.
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.
- Each group of two students should
obtain one egg and the necessary glassware from the
- Break the egg into a beaker. Add water
to the egg and stir before pouring down the drain.
- 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
- Dry the shell for about 10 minutes in
- Grind the shell to a fine powder in a
- The rest of this experiment is done
- 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.
- Add several drops of ethanol to each
flask. This acts as a wetting agent and helps the HCl
dissolve the CaC03.
- 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.
- 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.
- Add 3-4 drops of phenolphthalein
(a.k.a PhPL) to each flask.
- 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 +/-
- 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.
- Repeat the titration for the other two
- Calculate the percent calcium carbonate in each
sample and the mean value. Calculate the average deviation from the mean.
- Wash the egg residue out of the conical flask with
hot soapy water and a test tube brush.
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
- 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.
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
- Initial moles HCl = VHCl MHCl
= 0.0100 L x 1.00 mole/L = 1.00x10-2 mol
- moles HCl left = VNaOH MNaOH
= 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 =
- 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
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?