CHEMISTRY 26.1 Elementary Quantitative Inorganic Analysis EXPERIMENT 5: DETERMINATION OF THE SOLUBILITY PRODUCT CONSTANT OF CALCIUM HYDROXIDE N. CASTOR 1 1 INSTITUTE OF BIOLOGY, COLLEGE OF SCIENCE UNIVERSITY OF THE PHILIPPINES DILIMAN, QUEZON CITY 1101, PHILIPPINES DATE SUBMITTED: MARCH 4, 2015 DATE PERFORMED: FEBRUARY 20, 2015 _____________________________________________________________________________________________________________________________________________________ I. 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 is: The Ksp expression of the reaction, therefore, is: where Ksp is the solubility product constant. 2. What is the working equation for the determination of the [Ca 2+ ] in the saturated solution of Ca(OH)2? The working equation for the determination of the concentration of Calcium (II) ions in the saturated solution of Ca(OH)2 is 3. Write the Ksp of Ca(OH)2 in terms of its (a) molar solubility s, (b) [OH - ], and (c) [Ca 2+ ]. (a) (b) (c) 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. Common ions and ionic strength both have effects on the solubility of the precipitate of a salt like Ca(OH)2. Common ions have been observed to increase the solubility, which shifts the reaction towards the reactant side. This can be explained by Le Chatelier’s principle wherein the addition of common ions push the reaction to regain equilibrium by converting the newly added ions into the original precipitate. An increase in ionic strength also increased the solubility of the solution. This is because one of the factors of the measure of ionic strength is the concentration of the ions of the reactants. The higher the concentration of the ions of the reactants, the higher the solubility of the reaction would be because more moles of reactant would be able to react with each other. 5. What are the possible sources of errors and their effect on the calculated parameters? Rationalize. 1 Ca(OH) 2(s) ! Ca (aq) 2+ + 2OH (aq) − K sp = [Ca 2+ ][OH − ] 2 [Ca 2+ ] = [OH − ] 2 = M1 [OH − ] V1 [OH − ] V2 [OH − ] x 1 2 [K sp ] = 4s 3 [K sp ] = [OH − ] 3 2 [K sp ] = 4[Ca 2+ ] 3