Chemistry 208 – Winona State University – Spring 2004 – Dr. Nalli
Determination of Calcium Carbonate Content in Chicken Eggshells
Purpose: The purpose of this experiment is to use the ideal gas law to determine the percentage of
CaCO3 in chicken eggshells. You will also use a gravimetric (i.e., based on mass measurements)
method to determine the same quantity. Finally, a comparison of the accuracy and precision of the
two methods will be made.
Introduction: Calcium carbonate, which is found in bird eggshells and seashells, reacts with
hydrochloric acid according to the following equation:
CaCO3(s) + HCl(aq)

CaCl2(aq) + H2O(l) + CO2(g)
This reaction can be used to determine the mass percentage of CaCO3 in an eggshell sample. This
can be accomplished in two ways. The portion of the eggshell that is not CaCO3 does not react with
acid and remains as a solid. This solid can be filtered from the solution and weighed to give the
mass of non-CaCO3 material in the eggshell sample. Simple subtraction gives the weight of calcium
carbonate. The other method involves measuring the volume of CO2 produced and using the ideal
gas law (PV=nRT) to determine the moles of CO2 produced. The balanced chemical equation tell us
that, in theory, the number of moles of CaCO3 present in the sample is equal to the number of moles
of CO2 produced. (This assumes a 100% percent yield for the reaction.) Thus the volume of CO2
measured leads to a value for the moles of CaCO3 present in the eggshell. It is then a simple matter
to convert moles of CaCO3 to grams and calculate the percentage.
Methods Overview:
The volume of CO2 produced in the reaction is determined by measuring the volume of water
displaced in an inverted graduated cylinder. The number of moles of CO2 can be calculated from
this volume using the ideal gas law, if P and T are known. Thus you will also need to take
measurements of the temperature and the pressure. The pressure of the CO2 gas collected is equal to
the barometric pressure (measured using the barometer in the lab) minus the vapor pressure of water
at the measured temperature (look up using reference available in the lab).
Upon completion of the reaction, the remaining solid is vacuum filtered, dried, and weighed to give
the mass of non-CaCO3 material.
Experimental: Perform at least two trials on the eggshell samples available in the lab. Carefully
note your observations during all phases of the experiment on the observations sheet. The eggshell
should be reasonably dry and as much of the inner membrane as possible should be removed. It
will also be helpful to crush the eggshell into very small pieces. Use 0.3 g of eggshell and 15 mL of
3M HCl for each trial. After the reaction is complete as indicated by cessation of gas evolution, the
reaction solution is vacuum filtered using pre-weighed filter paper, washed with a small volume (12 mL) of cold water, transferred to a small pre-weighed beaker and dried for one week before the
final weighing.
The instructor will demonstrate the set up of the reaction/gas collection apparatus. The apparatus for
vacuum filtration will also be demonstrated but please read about it in advance at this website:
http://orgchem.colorado.edu/hndbksupport/filt/filtration.html. (Make sure you look at the part about
vacuum filtration, not the part about gravity filtration.)
Chemistry 208 – Winona State University – Spring 2004 – Dr. Nalli
Names __________________________________________________________
Data and Observations:
Data Table:
Trial 1
Eggshell sample
mass (g)
initial gas volume in
graduated cylinder
(mL)
final gas volume in
graduated cylinder
(mL)
volume of gas
formed (L)
Temperature (ºC)
Temperature (K)
Barometric Pressure
(torr)
H2O Vapor Pressure
(torr)
CO2 pressure (torr)
CO2 pressure (atm)
CO2 yield (moles)
(calculate using
PV=nRT)
CaCO3 in sample
(moles)
CaCO3 molar mass
(g/mol)
mass CaCO3 in
sample (g) (gas law
method)
% CaCO3 (gas law
method)
mass of solid
residue (g)
mass CaCO3 in
sample (g) (residue
weighing method)
% CaCO3 (residue
weighing method)
Trial 2
Trial 3
Chemistry 208 – Winona State University – Spring 2004 – Dr. Nalli
Names __________________________________________________________
Observations
Eggshells
Apparatus (diagram the reaction and gas collection apparatus)
Reaction
Filtration
Residue
Chemistry 208 – Winona State University – Spring 2004 – Dr. Nalli
Results and Discussion
Summary Table – Percent CaCO3 in Chicken Egg Shells
Method
Trial 1
Trial 2
Trial 3
Gas-Law
Average
Gravimetric
Literature*
Value:
Reference:
* It should be possible to find a literature value on the Web using a standard search engine such as
Google.
Questions/Discussion (neatly write in ink or type on a separate sheet)
(1) Discuss the accuracy and precision of the two methods. Which was the more precise method?
Which was more accurate?