To Determine the Percentage by Mass of Calcium Carbonate in Eggshells

1065 Words Oct 3rd, 2008 5 Pages
Aim
The purpose of this experiment is to determine the percentage by mass of calcium carbonate in eggshells.

Introduction
To avoid the breakage of eggs before reaching market, the eggshells needs to be as strong as possible. The strength of eggshells is mainly determined by the percentage of calcium carbonate in it. In order to monitor the quality of eggshells, the following experiment has to be done to determine the percentage of calcium carbonate in eggshells.
In this experiment, back titration is used. First, excess acid is reacted with the calcium carbonate in eggshells:
2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l)+ CO2(g) Later, if we can find out the number of mole of unreacted acid, number of mole of calcium carbonate can
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5. A 25 cm3 aliquot of the diluted eggshell solution was pipetted to a clean beaker. The beaker was placed on a magnetic stirrer and a pH meter was put into the solution.
6. 12.5 cm3 of 0.9992 M sodium hydroxide solution was transferred to a 250 cm3 volumetric flask by burette and de-ionized water was added to the mark. The flask was shaken to ensure homogeneity of the solution.
7. A clean burette was filled with the diluted sodium hydroxide solution (~ 0.05M). The initial burette reading, accurate to the nearest 0.05cm3, was recorded. During trial run, about 2-3 drops of methyl orange was added as an indicator.
8. Sodium hydroxide solution was run from the burette into the flask with the magnetic stirrer being turned on. When the indicator was turned slightly yellowish, sodium hydroxide was added dropwisely until the reading of the pH meter is 7.00. The final burette reading, accurate to the nearest 0.05cm3, was recorded.
9. The burette was refilled with the diluted sodium hydroxide solution. Steps 7 and 8 were repeated 3 times without using indicator. Sodium hydroxide was added dropwisely when the burette reading was close to the final reading in trial run.

Diagram

Results Tables
Trial 1 2 3
Burette reading / cm3 Final 26.80 26.85 26.80 26.75 Initial 0.00 0.00 0.00 0.00
Volume used (titre) / cm3 26.80 26.85 26.80 26.75
Mean titre / cm3 26.80

Calculation
The number of moles of NaOH used in each titration
=
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