Determining the Ksp of Calcium Hydroxide
Submitted to: Mrs. Rousseau
Submitted by: Alexandra
Couse: SCH 3U
Date: December 16th 2013
Purpose:
To identify which out of the two possible methods of determining the solubility product
constant of calcium hydroxide is the most accurate.
Hypothesis:
If one method provides a Ksp value closest to the accepted value, then it is the
most precise method. If the temperature and pressure are kept constant, then the titration
will be most accurate, because the moment at which the endpoint is reached can be easily
identified with the use of an acid-base indicator. It can be predicted that the Ksp value for
calcium hydroxide will be 5.02x10-6.
Abstract:
The purpose of this lab was to use the two methods of titration and serial
dilution to find the Ksp of calcium hydroxide. The method that provided a Ksp
value closest to the accepted value was thus proven most exact. The result
obtained from this experiment was that the serial dilution was the most
accurate, given it provided a Ksp value of 7.81x10-6. This differed by a value
of experimental error of ~56% from the actual value, being 5.02x10-6.
Materials: Determining the Ksp of Calcium Hydroxide Using a Serial Dilution
Lab apron
0.1 mol/L calcium
nitrate, Ca(NO3)2(s)
Minimum 24-well
spotplate (12 x2)
Distilled water
Eye protection
0.1 mol/L potassium
hydroxide, KOH(s)
4 pipets
Dark coloured paper
Procedure:
Part 1: Determining the Ksp of Calcium Hydroxide Using a Serial Dilution
1. A spotplate was placed on a dark sheet of paper.
2. 5 drops of distilled water were added to each of 11 consecutive wells in row A of
the spotplate, leaving the first well, A1, empty.
3. 10 drops of 0.1mol/L calcium nitrate were added to well A1.
4. The solution from A1 was drawn into an empty pipet and 5 drops of the solution
were placed into A2. Any excess solution was returned to A1.
5. Using the same pipet, 5 drops of the solution in A2 were transferred to the water
in A3.
6. Step 5 was repeated for each of the remaining wells. The 5 drops from well A12
were discarded into a sink with lots of running water.
7. A new pipet was used to place 5 drops of 0.1mol/L potassium hydroxide into
wells B1 through B12.
8. A clean pipet was used to transfer the entire contents of well A2 into B2, A3 into
B3, etc., until A12 and B12 were mixed.
9. All of the wells were examined. The first well that appears to have no precipitate
was identified.
10. Chemicals were disposed into the Chemical Wastes container. All materials were
returned to their proper location. Hands were washed thoroughly with soap and
water.
Observations/Results:
Table 1: Observations and Calculations for serial dilution
Well #
A initial
B initial
C final
D final
2+
2+
[Ca (aq)]
[OH (aq)]
[Ca (aq)]
[OH-(ap)]
(M)
(M)
(M)
(M)
1
0.1
0.1
0.05
0.05
2
0.05
0.1
0.025
0.05
3
0.025
0.1
0.0125
0.05
-3
4
0.0125
0.1
6.25x10
0.05
5
6.25x10-3 0.1
3.125x10-3 0.05
6
3.125x10-3 0.1
1.56x10-3 0.05
-3
7
1.56x10
0.1
7.81x10-4 0.05
8
7.81x10-4 0.1
3.91x10-4 0.05
-4
9
3.91x10
0.1
1.95x10-4 0.05
10
1.95x10-4 0.1
9.77x10-5 0.05
2
E (C ×D2)
ion
product
1.25×10-4
6.25×10-5
3.125×105
1.56×10-5
7.81×10-6
3.91×10-6
1.95×10-6
9.77×10-7
4.88×10-7
2.44×10-7
Precipitate
Yes
Yes
Yes
Yes
No
No
No
No
No
No
11
12
9.77x10-5
4.88x10-5
0.1
0.1
4.88x10-5
2.44x10-5
0.05
0.05
1.22×10-7
6.10×10-8
No
No
Sample Calculations:
1. Part 1: Serial Dilution Calculations
To make 0.1 M Ca(NO3)2:
(0.1mol/L)×(164.1g/mol)×(0.1mL)=1.641g/100mL Ca(NO3)2
To make 0.1 M KOH:
(0.1mol/L)×(56.11g/1mol)×(0.1mL)=0.5611g/100mL KOH
To calculate the initial concentration of Ca2+:
C1V1=C2V2
0.1M×5 drops Ca(NO3)2 = C2×10 drops
C2=0.05M
Chemical Reactions:
Part 1:
Ca(NO3)2 (aq) + 2KOH (aq) → Ca(OH)2 (s) + 2KNO3 (aq)
Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)
Part 2:
Ca(OH)2 (aq) + 2HCl (aq) → CaCl2 (aq) + 2H2O (l)
Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)
Analysis:
In attempt to find the solubility product constant for calcium hydroxide, two
methods were endured. The first method was that of a serial dilution, where the presence
of a precipitate in a spotplate was monitored after consecutive dilutions. The second was
a titration against hydrochloric acid, where the endpoint was reached and indicated using
the acid base indicator, bromothymol blue. By comparing both methods, it was proven
that the serial dilution provided the most accurate Ksp value of 7.81x10-6. This was
evident seeing as it was the closest to the accepted value obtained from the Handbook of
Chemistry and Physics, 5.02x10-6.
The results obtained justify that the hypothesis aforementioned was incorrect. It
was predicted that the titration would provide a more exact Ksp value, but after each part
of the procedure was elaborated and performed, the opposite was proven. The titration
concluded to give a Ksp value of 5.89x10-7 and the method of serial diluting contributed a
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Ksp value of 7.81x10-6. The value of 7.81x10-6 was closer to the accepted value, with only
55.6% error in comparison to the 88.2% that the titration possessed, and hence proven
most exact.
In both sections of the lab, the Ksp values presented some ambiguities that resulted
in these values of experimental error. These ambiguities likely occurred because of
experimental errors caused by the limitations of the equipment used. Some sources of
error and uncertainty in the experimental design that may have compromised the obtained
results involve dirty glassware and cross contamination between solutions. Additional
sources of error include inaccurate measurements from the electronic balance, improper
use of pipette, inconsistent amounts of solutions in each well of the spot plate and failure
to indicate the exact moment at which the endpoint was reached with the naked eye.
During both parts of the lab, contaminated glassware and salt solutions may have
affected results by altering concentrations of substances and affecting their basicity or
acidity. When making the potassium hydroxide and calcium nitrate solutions, the
electronic balance was used. Given it only measured to two decimal places, there existed
some inaccuracy in the overall amount of salt weighed, which could have compromised
the final results obtained.
During the serial dilution, the pipette was used to transfer five drops from one
well into the next consecutive well. This allowed for inconsistency pertaining to the
overall amount of solution in each well, because the sizes of the drops that the pipette
produced were not always the same. This in turn may have affected the concentration of
each well, thus affecting the trial ion product calculated.
During the titration, the pH changed very quickly, therefore making it quite
difficult to approximate the exact moment at which the solution became neutralized. This
accounts for a source of error, because the moment at which a colour change was
visualized and the stopcock was closed could have been slightly past or before the actual
endpoint. This makes it difficult to know just how much HCl it took to reach the
endpoint, which affects the concentrations used to calculate the solubility product
constant.
In the future, these sources of error should be avoided by thoroughly rinsing all
glassware before using it and by using new beakers and pipettes for each different
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substance to avoid cross contamination. Additionally, one should invest in an electric
balance that weighs up to four decimal places and use a pipette or medicine dropper that
is accurate and consistent with the size of the drops it produces. Lastly, once one sees that
the colour of a solution begins to change colour during a titration, the stopcock should be
kept only slightly open to allow the solution in the burette to enter the retrieving beaker
drop by drop. This avoids one from overshooting the endpoint.
The methods of titrating and serial diluting are both very useful and important in
chemistry, as they enable one to find the concentrations of unknown solutions (Hans van
Kessel et al., 2003). These methods can be applied to medicine and research in attempt to
better the lives of others.
Finding the right medication, or more importantly, the right dose of medication,
plays a crucial role in helping children diagnosed with ADHD. Every medication works
different for every child and doctors must assure that when prescribing drugs for their
patients, they are choosing one that is most effective and has minimal side effects. It is
through the method of titration that pharmacists and doctors are able to achieve this
balance. Children start treatment with a very low dose of medication and gradually work
up to higher and higher doses (Quinn, 2012).
Serial dilutions have also had a positive impact on medicine; Doctors will use
serial dilutions for patients with extremely elevated blood sugars, known as Diabetic
Ketoacidosis. The serial dilutions work to indicate blood glucose levels and to determine
the quantity of units of insulin required to be administered intravenously per hour.
Conclusion:
In comparing the accuracy of two methods used to find the solubility product
constant of calcium hydroxide, it was concluded that the method of serial diluting was
most accurate. The serial dilution performed provided a Ksp value of 7.81x10-6.
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References
Kessel, H. V., Jenkins, F., Davies, L., Plumb, D., Di Giuseppe, M., Lantz, O., &
Tompkins, D. (2003). Nelson Chemistry 12. Nelson, a division of Thomson
Canada Limited.
Östman, M. (2007). Calcium Hydroxide. Retrieved December 2, 2013, from Sweedish
Chemical's Agency- Information on Substances website:
http://apps.kemi.se/flodessok/floden/kemamne_eng/kalciumhydroxid_eng.htm
Quinn, MD, P. (Ed.). (2012). ADHD Medication Titration. Retrieved December 15,
2013, from Web MD website: http://www.webmd.com/add-adhd/childhoodadhd/adhd-medication-titration
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