CALCIUM IN SHELLS

1. Describe the type of error and how you tried to minimize or avoid the error.
In order to get good results, it is important to define the research question properly. It is necessary to decide if one wants to generalize the sample and thus include many different shells from various regions or use a more for a specific sample that is representative only for one defined region. Also one needs to decide if a comparison between groups is wanted; the sampling procedure has to be applied accordingly.
One has to take the best representative sample, pick for example the most common shell of a region, since calcium levels could vary.
The shells have to be crushed properly and moreover one has to take care of segregation of particles and homogenization. Another important aspect is to clean all of the materials used in this experiment.

2. What reaction takes place when you dissolve CaCO3 with HCl?
CaCO3 + 2HCl → Ca2+ + 2Cl- + H2O + CO2

3. Supposing the shells are 100% pure CaCO3, what is the Ca molarity (mol/L) of the 100 mL solution? What is the concentration in ppm?
According to the table: CaCO3= 50 grams/mol and the proportion of Ca is 40% of the total amount. 0.5 grams of Ca (the used sample) * 40% = 0.2 grams of Ca.
In 100 ml the Ca concentration is 200/0.1 L = 2000 ppm.

4. How many moles of Ca are present in the 20 mL solution that you titrate, supposing 100% pure calcium carbonate shells?
Concentration of Ca in 20ml:
Moles of Ca = 0.2 grams / 40g/mol = 0.005mol.
Concentration = 0.005mol / 0.1L = 0.05 M.

In 20 ml solution: M2 = 0.05 (mol/l) * 0.02L = 0.001 mol

5. How many mL of 0.1M EDTA would you theoretically have to use, supposing 100% pure calciumcarbonate shells.
1 mol of EDTA reacts with one mol of Ca. There is 0.001 mol Ca in 20 ml solution thus it will need 0.001 mol EDTA for the reaction to take place.

EDTA = 0.1M 0.001/0.1 = 10mL

6. How many mL EDTA did you actually use, if it deviates from the theoretical pure calcite titration value, explain why.
One titration for each sample was done and we used an average of 9.5 ml of EDTA.
The possible problems that might influence our results are:
- Shells do not contain 100% CaCO3
- Errors during the experiment such as: pipetting errors or leakage of the solution
- The change in color of the indicator was not very clear to the experiment as a result of this there might have been used too much of EDTA.
- There might have been errors in the molarity of EDTA which would have also change the results.

7. Evaluate the precision of your titration.
Before using the titration apparatus we cleaned it twice thus we assume that the titration apparatus was sufficiently clean. Problems occurred with the addition of the small droplets of EDTA and sometimes the added quantities were bigger than a drop. Thus, the measures were not very exact.

8. What is the Ca concentration in the shells based on the titration results.
9.5 mL 0.1 M EDTA contains 0.00095 moles. This results in 0.00095 moles of Ca included in the sample. It means that 0.00475 moles of Ca and the same number of moles of CaCO3 were contained in 100 mL of the solution. We dissolved 0.5 grams of shells in the solution so, in case of 100% purity, 0.005 moles of CaCO3 are expected in the solutions. Thus, the Ca concentration in the shells equals 95 percent.


9. Draw the calibration curve for Ca measured on the AAS.
Ppm of Ca Abs
0 0
2 0.012
4 0.024
8 0.051

10. How did you dilute your sample for AAS measurement?
To get a solution of 100ppm we got 10 ml of 1000ppm and 90ml of distilled water.
Therefore, from this solution to obtain a solution of 2ppm it is used 0.1ml of 100ppm stock solution and 49.9 ml diluted water.
Then to get a solution of 4 ppm it is used 0.2ml of the stock and 49, 8 ml of distilled water and for 8ppm it is used 0.4ml of the stock and 49.6ml of distilled water.


11. What is the concentration in the shells based on the AAS measurement?
The average absorption value was 0.02175 for our samples. The regression curve equation, computed by Microsoft Excel, was following: y = 0.0063x. According to this our sample showed an average ppm value of about 3.45. This means that our sample consisted of a 1725 ppm. The calcium concentration measured was 86.25 percent. This value is too high. It seems like there is Ca that is not part of CaCO3 but exists additionally. Another explanation could be that we used more than 0.5g of shells to begin with.

12. Assuming the value given by AAS is correct, what is the average error (%) of the titrations?
The average error is 1500/1725 *100= 86.96%.

Data Analysis: Acid Rain Simulation

Test Tube A: Mg Ribbon Test Tube B:CaC03 Test Tube C:Apple Skin
pH before reaction 2.14 2.16 2.17
pH after reaction 2.24 6.19 2.23
pH 0.10 4.03 0.06