For the next few steps your setup should look like this:
- Weigh accurately 0.5g of the power in a conical flask.
- Use a mortar and pestle to powder the eggshell. Powdering will increase the rate of reaction and will result in the time for the experiment to occur to reduce.
-
Using a graduated pipette add 20.0cm3 of 1.00moldm-3 of hydrochloric acid slowly any swirl the flask to prevent any small amount from escaping from the flask with the carbon dioxide that is produced.
-
Once the reaction is completed add about 20 cm3 of distilled water and transfer all the contents of the flask to a 100 cm3 of measuring cylinder. Add more water to it till the final volume of the solution is 100 cm3.
-
Take in batches of 10.0 cm3 the solution and transfer it to a conical flask. Add two drops of phenolphthalein to the solution.
-
In a burette add 0.100moldm-3 of sodium hydroxide.
- Now titrate the solution. Add sodium hydroxide solution at a very slow rate, at the same time keep shaking the conical flask so that the phenolphthalein that shows the pink colour is quickly dissipated.
- A point will be reached when the solution will become completely pink and the shaking of the conical flask will not dissipate the colour. At this point stop the flow of sodium hydroxide.
- Note down the reading that is shown on the burette.
Data collection And Processing
The following table shows the data that was collected:
The average of sodium hydroxide that was used to neutralize the hydrochloric acid is:
The error in the average value is:
The process of back titration can be used to find out the mass of
The two reactions that occur during the process are:
We can see from the above equation that
of
reacts with
of
Finding the number of moles of sodium hydroxide:
Therefore:
The total amount of
that was taken is:
The error here is
as the least count on a
measuring cylinder is
and the final error is
Now the amount of
that reacted with
can be calculated by:
of
reacts with
of
therefore
of
will react with
The value is
Now moving on to calculate the percentage purity of
in eggshell:
The error in the percentage purity is:
This error is a great error and can be relied upon as the original calculated value of the percentage of Calcium carbonate that is present in an eggshell is near
.
The original value of percentage purity of calcium carbonate in eggshell is near to
. Therefore the deviation from the original value that I have in my experiment is:
According to my readings the percentage purity of Calcium Carbonate in eggshell is:
Conclusion and Evaluation
The percentage purity of Calcium Carbonate in eggshell is
.
This error can be accounted for the uncertainty that the apparatus that I used in the laboratory at the same time manual errors that were caused. Manual errors that were caused include:
- Parallax error. This was caused when the readings from the burette has to be taken. The lower meniscus of the measurement has to be taken. Due to the height of the burette there were some errors in the reading. At the same time sometimes the value lied between two different marks on the burette this led to taking the lower value into consideration and thus the error increased.
-
It was noticed that even after well mixing of the solution of the acid and the eggshell some residue was left in the solution. The correct value of the percentage of calcium carbonate was not known earlier, therefore it was taken that all the calcium carbonate had dissolved and hence the leftover was the impure substance.
- Sometimes during the titration a bit extra phenolphthalein was added. This increased the error in the experiment even further.
There were six reading taken and an individual count of all the samples was not taken but instead an average was taken. This again results in an increase in the error in the experiment and thus this is the flaw that is present in the procedure. When the reacted solution of the salt and the acid is diluted the concentration of the unreacted acid decreases and the error is increased even further. The most significant error in the percentage purity was caused by the
measuring cylinder. As every time a measure of
of
was taken it was not accurate and this resulted in the varation in the amount of
that was neutralized each time.
The improvements that can be added to the experiments will be:
- Using a dropper to measure an accurate amount of the solution so there is a minimum of variation in the calculated values.
Titrating about
- of solution as it is easier to isolate form the total solution and at the same time the results will improve and there will be a reduction in the uncertainty of the experiment.
The titration should be carried our very carefully so that excess of
- is not used and thus the result will be more accurate.
With these improvements one should attain a more accurate result.
