DETERMINATION OF THE SOLUBILITY PRODUCT CONSTANT OF CALCIUM HYDROXIDE B.C. HUELGAS1 and R. CALUBAD2 1INSTITUTE OF BIOLOGY, COLLEGE OF SCIENCE 2INSTITUTE OF BIOLOGY, COLLEGE OF SCIENCE UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY 1101, PHILIPPINES DATE SUBMITTED: 4 MARCH 2015 DATE PERFORMED: 20 FEBRUARY 2015

ANSWERS TO QUESTIONS 1. Write the net ionic reaction for the precipitation of Ca(OH)2 and give the Ksp expression of the reaction.

The net ionic reaction for the precipitation of Ca(OH)2 can be expressed as,

Ca(OH)2(s) Ca2+ + 2OH-

For this equation, the Ksp expression of the reaction is,

Ksp = [Ca2+] [OH-]2 [1]

2. What is the working equation for the determination of the [Ca2+] in the saturated solution of Ca(OH)2?

The working equation for the determination of the [Ca2+] in the saturated solution of Ca(OH)2 can be represented by,

[Ca2+] = 2[OH]

or

[Ca2+] = 2(V ianalyte)

(V titrant)[H+]

3. Write the Ksp of Ca(OH)2 in terms of its (a) molar solubility s, (b) [OH

-] and (c) [Ca2+]. The Ksp of Ca(OH)2 written in terms of its molar solubility is

sKsp = 4 3 in terms of [OH2-],

( )4 2[OH ] 3

and in terms of [Ca2+]

[Ca ]4 2+3

[2]

4. Discuss the effects of common ions and increasing ionic strength on the solubility of the Ca(OH)2 precipitate. Do they coincide with theoretical effects? Explain.

The presence of another common ion in the solution can reduce the solubility of the precipitate, shifting the equilibrium of it back to the reactant side [3]. In general, the solubility of a slightly soluble salt is decreased by the presence of a second solute the furnishes a common ion [4]. However, in the data gathered it can be seen that the opposite was observed.

On the other hand, the data obtained did not have a definite trend with respect to the effect of ionic strength. The data at first decreased, from to 0.010 M to 0.050 M, then from this point it gradually increased starting from 0.050 M to 0.50 M. This data however, do not coincide with the diverse-ion theory, which deals with inter-ionic contacts of the ions present in a solution. It states thatthe interactions between ions become stronger as the ionic concentration of the solution rises. The increase in attractions leads to lesser activities of the participating ions or their effective concentration thus causing an increase in solubility [5].

5. What are the possible sources of errors and their effect on the calculated parameters? Rationalize.

Possible sources of errors are improper measurement of the reagents. If the concentrations of the reagents are not accurate, then it will change the results obtained since it depends upon the concentration of the substance. Another source of error may have been from the preparation of the sodium carbonate suspension. The suspension may not have been saturated enough before an analyte was measured out of it. This will result to lesser Ksp than the theoretical. Last possible cause would have been from the titration process. Improper way of titrating, wrong measurement of the used volume of titrant etc., will all lead to a gross error on the calculation of the [OH-] and may cause failure of the experiment.

REFERENCES [1] UPD Analytical Chemistry Group (ACAG).Analytical Chemistry Laboratory Manual.Quezon

City: Institute of Chemistry, University of the Philippines, 2013 ed.. Print. [2] Zumdahl, Steven S., and Susan A. Zumdahl. Chemistry. Belmont, CA: Brooks/Cole, Cengage

Learning, 2014. Print. [3] Brown, Theodore L., H. Eugene LeMay, and Bruce Edward. Bursten. Chemistry: The Central

Science. Upper Saddle River, NJ: Prentice Hall, 2000. Print. [4] Chang, Raymond, and Kenneth A. Goldsby. Chemistry. New York: McGraw-Hill, 2013. Print. [5] Petrucci, Ralph H., and William S. Harwood. General Chemistry: Principles and Modern Applications. 8th ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Print.