Calcium Benzoate Solubility posted by Alexander James Krzyston

Happiness by Alexander J Krzyston

Introduction:
    In this lab we sought to determine the solubility product of Calcium Benzoate.  We did this by titrating calcium benzoate against EDTA standard solution.


When the EDTA is added to the calcium benzoate solution, it will react with Ca2+.  Thus by knowing the endpoint of the solution, we can calculate the final concentration of Ca2+ in solution (it will be equal to the volume of EDTA used in titration). Since calcium and benzoate are in a 1:2 ratio, the final concentration of Bz- will be double the change in [Ca2+] concentration.  By knowing the final concentration of both Ca2+ and Bz-, we can calculate the solubility product of CaBz2:
[Ca2+][Bz-]2 = Ksp

Alexander Krzyston Calcium Benzoate| Alexander J Krzyston Calcium Benzoate| Alex James Krzyston
Alex Krzyston Calcium Benzoate| Alex J Krzyston Calcium Benzoate| Alexander James Krzyston
NORTHWESTERN UNIVERSITY Calcium Benzoate| EVANSTON Calcium Benzoate| BURR RIDGE
Overview:
In this lab source of error could be due to the fact that the specific end point of the solution was rather difficult to determine.  This is because the color change was from red to purple to blue, and the difference between purple and blue can be rather objective and difficult to differentiate purple from blue when the two are mixed in solution.  As a result, it would be easy to stop before the end as the solution begins to turn bluish, but is not really totally blue yet.  A way to avoid this problem would be to have an over titrated solution as reference to compare the two solutions to ensure that you do not stop titrating too early.  Although we did have the first practice titration to do this, if you under titrated that solution you would not know what the actual color would be and all the other titration would also be under titrated.  This would result in a lower final concentration of Ca2+ ions and a higher final concentration of Bz- ions.  Also, because the blue was difficult to see when mixed with the purple color during titration, it would also be easy to over titrated. 
Another source of error in this lab would come from the EDTA solution.  If you messed up on the preparation of the EDTA solution initially, then all of the titrations would be wring because that was the solution we were using to titrated. 
Perhaps a way to improve this lab or an interesting variation would be to determine the solubility product of Calcium Benzoate by filtering out the precipitate from the over saturated solution and then measuring that value and use it to determine the solubility product and the final concentration of the two ions.   

Procedure:
    Procedure was same as given in lab manual
*as oppose to adding the MgCl2(6H2O) salt to each CaBz2 solution to sharpen the end point, we added the MgCl2(6H2O) to the standardized Na2H2E solution.  By doing this the slat only had to be added once as oppose to six separate times.

Data:
Weight of Na2H2E powder: 8.8986g
Volume of Na2H2E solution: 250mL
Solution #    T(oC)     [Ca2+]i     [Bz-]i    Vi(mL)    Vf (mL)    ΔV (mL)    Ksp
1     19.00     0.070     0.320      0.47     12.10     11.63   
1     19.00     0.070     0.320     12.10     23.70     11.60   
2     19.00     0.200     0.200      3.89     41.75     37.77    
2     19.00     0.200     0.200      0.22     37.94     37.72   
3     19.00     0.125     0.250      0.60     21.89     21.29   
3     19.00     0.125      0.250     21.89     43.20     21.31   
    *all concentrations in molarity (mol/L)
Analysis:
1. concentration of [Na2H2E]:
    0.09562 M

2. final concentration of [Ca2+]f
    See table for values
    See attached sheet for calculations

3. final concentration of [Bz-]f
See table for values
    See attached sheet for calculations

4. Ksp
See table for values
    See attached sheet for calculations

Solution #    [Ca2+]i     [Bz-]i    [Ca2+]f     [Bz-]f    Ksp
1     0.070     0.320     0.0556     0.0555     0.00474
1     0.070     0.320     0.292     0.290     0.00467
2     0.200     0.200     0.181     0.162     0.00475
2     0.200     0.200     0.180     0.160     0.00461
3     0.125     0.250     0.102     0.204     0.00424
3     0.125      0.250     0.102     0.204      0.00424
    *all concentrations in molarity (mol/L)
5. Mean Value of  Ksp:
    0.00424
        *See attached sheet for calculations

6. Recalculation of Ksp at 20oC:
    (Starting at Ksp value at 19oC and based on 2.5% increase per degree)

    0.00465
        *See attached sheet for calculations

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