Solubility Product Constant and Common-Ion Effect

Experiment (13) Solubility Product Constant And Common-Ion Effect Objectives: 1- To determine the value of the solubility product constant, Ksp for Ca(OH)2. 2- To study the effect of common ions on the solubility of Ca(OH)2. Apparatus and Chemicals: 125-mL Erlenmeyer flasks (3) Saturated solution of Ca(OH)2. 50-mL buret (3 g per 100ml H2O). 25-mL piper (3 g of Ca(OH)2 and 1 g of CaCl2.2H2O per 100mL H2O). 0.1M HCL Methyl orange indicator. Introduction: Slightly soluble salts are those that have limited solubility in water. This experiment is concerned with heterogeneous equilibrai of slightly soluble salts. In order for a true equilibrium to exist between a solid and solution, the solution must be saturated. Calcium hydroxide is a slightly soluble base, and in a saturated solution a dynamic equilibrium exists: And the equilibrium constant is: The concentration of Ca(OH)2 in the denominator is constant because the concentration of a pure solid is constant, and be combined with Kc to give a new constant, Ksp which is called the solubility product constant: From the equilibrium of Ca(OH)2, the molar solubility, x, equal; At a given temperature the value of Ksp is constant. Ksp can be determined from measuring the solubility of the substance in water. Example (1): If 2.42 x 10-4 g of BaSO4 is dissolved in 100mL of water. Calculate Ksp of BaSO4? From the equilibrium: Example (2): 50.0mL of a saturated solution of Ca(OH)2 required 11.0mL of 0.1M HCL to reach the methyl orange and point at 25oC. What is the value of Ksp for Ca(OH)2. The Common Ion Effect: As we mentioned earlier, in saturated solutions of slightly soluble salts there is a dynamic equilibrium between the solid salt and its dissolved ions. The addition of an ion common to the salt will shit the equilibrium to the left and the solubility of the salt decreases, this according to Le Chatelier principle. Example (3): What is the molar solubility of AgCl in water and in a 0.010 M solution of NaCl? Ksp AgCl is 1.7 x 10-10. in pare water Molar solubility In a 0.01 M solution of . Molar solubility So the solubility of a salt is less in a solution that already contains one of its ions than it is in pure water. In this experiment you will determine the solubility product constant for Ca(OH)2 by titrating OH- with a standard HCL solution. Also, the molar solubility of Ca(OH)2 is to be determined in the presence the common ion, Ca2+. Procedure: Two trials are required, follow this procedure for each trial. A- Determination of Ksp and molar solubility of Ca(OH)2: 1- Decant the settled saturated Ca(OH)2 solution into a 125-mL Erlenmeyer flask. Rinse 25-mL pipet with 2 mL of this solution, and transfer 25.0 mL of the solution using the pipet into a clean 125-mL Erlenmeyer flask. Add 2 drops of methyl orange indicator and measure the temperature of the solution. 2- Rinse a clean 50-mL buret three times with 5 mL portions of 0.1M HCl solution, then fill the buret with the acid. 3- Titrate the Ca(OH)2 solution prepared in step 1 with the 0.1M MCl solution, the color of the indicator will change from yellow to red at the end point. Record the volume of HCl used to reach this point. 4- Repeat titration with an new 25.0mL sample of Ca(OH)2 solution. B- Solubility of Ca(OH)2 in presence of Ca2+, the common ion effect: In this part you will repeat the four steps as in part A except that in step 1 you are going to use a mixture of Ca(OH)2 and CaCl2 solution. This mixture will be prepared for you. Data and Results: Trail 1 Trial 2 1. Concentration of standard HCl Solution (M) ………… ………… 2. Buret reading, initial (mL) ………… ………… 3. Buret reading, final (mL) ………… ………… 4. Volume of 0.1 M HCl used (mL) ………… ………… 5. Volume of saturated Ca(OH)2 solution (mL) ………… ………… 6. Temperature of saturated solution (oC) ………… ………… ………… ………… 8. [Ca+2] at equilibrium (M) ………… ………… 9. Ksp = [Ca+2] [OH-]2 ………… ………… 10. Average Ksp at _____________ oC ………… ………… 11. Average deviation ………… ………… 12. Molar solubility of Ca(OH)2 at __ oC = point (8) ………… ………… Trail 1 Trial 2 1. Concentration of standard HCl Solution (M) ………… ………… 2. Buret reading, initial (mL) ………… ………… 3. Buret reading, final (mL) ………… ………… 4. Volume of 0.1 M HCl used (mL) ………… ………… 5. Volume of Ca(OH)2 titrated (mL) ………… ………… 6. Temperature of solution (oC) ………… ………… 7. [OH-] at equilibrium (M) ………… ………… 8. Molar solubility of Ca(OH)2 at _________ oC ………… ………… 9. Average molar solubility of Ca(OH)2 ………… ………… 10. Average deviation ………… ………… A- Ksp and molar solubility of Ca(OH)2 7. [ ] at equilibrium (M) B- Solubility of Ca(OH)2 in presence of Ca+2