Find percentage of calcium carbonate in eggshells

A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Find percentage of calcium carbonate in eggshells
Is eggshell pure calcium carbonate? The shells of birds’ eggs are made from calcium carbonate. But is it pure? In this experiment, you will react eggshells with hydrochloric acid and try to find how much of the shell is calcium carbonate.
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Method:

1. Take a sample of dry eggshell and break it into small pieces.
2. Weigh a 100 cm³ beaker and then add the broken eggshell to it.
3. Record the new mass and work out the weight of the eggshell.
4. Carefully add 25 cm³ of hydrochloric acid and again record the weight of the beaker and contents.
5. Leave the beaker until the reaction stops and the eggshell has dissolved.
6. Record the new weight of the beaker and contents.

Calculations

1. How much mass did the beaker and contents lose?
2. Which compound in the equation is responsible for this loss?
3. Using M_r for this compound, how many moles of it were lost?
4. Using the equation, how many moles of calcium carbonate were dissolved?
5. What is the mass of this number of moles of calcium carbonate?
6. Using this figure and the original mass of eggshell, calculate the percentage by mass of calcium carbonate in eggshell.

Questions

1. What are the most likely sources of error in this experiment?
2. How could you try to get a more accurate result?
3. What assumption have you made about the composition of the egg shell that could have a serious effect on your result if the assumption is not true?

Answers

• Show Answers
  1. The egg shell was not fully cleaned and there could be other dirt or organic matter stuck to it. The eggshell was not fully dried before the experiment. The eggshell powder did not dissolve completely.
  2. Repeat the experiment several times and obtain the average.
  3. There are no other chemicals in the egg shell that will react with the HCl.

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