Determination of Calcium Carbonate in Eggshells
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)
(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 completion (endpoint). Instead, the
determination is achieved by adding an excess of acid to dissolve
all of the CaCO3 and then titrating the remaining excess H3O+
with NaOH solution to determine the amount of acid that did not react 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 that is used by the CaCO3. From
this, the amount of CaCO3 in the sample can be calculated.
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 (~$75) and brush |
- Each group of two students should
obtain one egg and the necessary glassware from the cart.
- 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. If you leave the membranes, the proteins in the membrane will react with the sodium
hydroxide and give poor results. Dry your eggshell with a paper towel and put into a labeled
beaker. Wash your hands with soap and water.
- Dry the shell for about 10 minutes in
- Grind the shell to a fine powder with a mortar and
The rest of this experiment is done individually.
Each person does their own analyses.
- Accurately weigh between 0.450 and 0.550
g of dried shell into each of 3 labeled 250mL conical
flasks. Be certain you record the exact mass of shell in 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 CaCO3.
- Pour about 40 mL of 1.0M HCl into a
beaker. Using a pipet to transfer10.0 mL of the 1.0M HCl to each 250mL flask
containing eggshell. Swirl the flasks to wet all of
the solid. Any HCl that you do not use should be disposed of in the
sink and diluted with plenty of water.
- Heat the solutions in the 250 mL flasks on a hotplate
they just begin to boil and then allow them to cool. Do not let them
boil dry! Rinse the walls of
the flask with water from your wash bottle.
- Add 3-4 drops of phenolphthalein indicator to each flask.
- Fill a beaker with about 100 mL of 0.100 M sodium
hydroxide. Using a funnel, a few mL of this solution to a clean
buret to rinse it. Empty the
buret into the sink. Fill the buret with the NaOH solution to just above
the top mark. . 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.
- 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 (or by half drops) until the color remains for at least 30 sec.
Read and record the final volume to ±0.01 mL. The volume added is
the difference between the initial and final readings (to ±
- Repeat the titration for the other two
- In your Results section, calculate the percent calcium carbonate in each
sample and the average value to report in your Conclusion. Be sure
you show all of your calculations, with units!
- Wash the egg residue out of the conical flask with
hot soapy water and a test tube brush.
Put your data and results in a neat table. Show one
complete calculation for the % calcium carbonate. Show the calculation for
Calculations (Show each
step clearly using units and keeping only the significant figures)
1. Calculate the number of moles of HCl
added to each shell sample. This is given by the expression:
|moles HCl added =
(0.0100 L HCl)*(1.00
moles HCl/liter) = 0.01000 moles HCl added
- 2. Calculate the moles of HCl left in
each sample after the reaction with CaC03.
|moles HCl left = (L NaOH
you used)*(0.100 moles NaOH/liter)*(1 mole
HCl/1 mole NaOH)
- 3. For each sample determine the
number of moles of HCl that reacted with CaCO3
by taking the difference between the moles of HCl added
and the moles of HCl remaining after the reaction is
|moles HCl reacted = moles
HCl added - moles HCl left
- 4. The moles of CaCO3 in
each sample is calculated by:
|moles CaCO3 = (moles
HCl reacted)*(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
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 =
Moles HCl left = VNaOH MNaOH =
0.02970 L x 0.100 mole/L = 0.00297 mol.
- Moles CaCO3 = moles HCl
reacted x (1 mol CaCO3 / 2 mol HCl )
- Moles HCl reacted = 0.0100 - 0.00297 =
- Moles CaCO3 = 0.0070 moles
HCl x (1 mol CaCO3 / 2 mol HCl ) = 0.0035 moles
- % 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 were the sources of error in this experiment? Why did you
put the eggshells in the oven? Are eggshells a good source of
lasted edited by jim byrd on 01/20/2006