Solubility Product

Solubility Product - 1 Solubility Constant DETERMINING THE...

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1 Solubility Constant DETERMINING THE SOLUBILITY The solubility of Ca(IO 3 ) 2 will be determined using an indirect redox titration. The IO 3 - ion is an oxidizing agent. It reacts with iodide ions, I - , to produce iodine molecules, I 2 . The iodine produced is then titrated with a standardized sodium thiosulfate solution, Na 2 S 2 O 3 . This is an iodometric titration , a versatile method for experimentally measuring the amount of an oxidizing agent in a substance or solution. Iodine in water, particularly when there are I - ions present, gives a deep reddish brown to a pale yellow color to the solution, depending on its concentration. If a starch solution is added to the iodine solution when the concentration of iodine is low, and the solution appears yellow, the iodine and the starch form an intensely blue colored complex. The disappearance of the blue color as sodium thiosulfate is added gives the endpoint in the titration. The reactions that take place are: IO 3 - (aq) + 5I - (aq) + 6H + (aq) 3I 2 (aq) + 3H 2 O(l) (4) and then I 2 (aq) + 2S 2 O 3 2- (aq) 2I - (aq) + S 4 O 6 2- (aq) (5) In reaction (4), the I - is provided by adding solid KI in excess, and the H + is provided by adding HCl solution. In reaction (4), the I 2 produced comes from the IO 3 - present. I - alone will not produce I 2 , except by the slow oxidation caused by the oxygen in the air. There are 3 moles of I 2 produced for every 1 mole of IO 3 - present. In reaction (5), 2 moles of S 2 O 3 2- are required for every mole of I 2 present. The net result is that for every 6 moles of S 2 O 3 2- used in the titration, there must have been 1 mole of IO 3 - originally present. The sodium thiosulfate solutions have been standardized. You will use the molarity of the thiosulfate solution to calculate the molarity of the iodate in the calcium iodate solutions. In each titration, you will add 10.0 ml of the calcium iodate solution to a flask using a pipet. The sodium thiosulfate solution will be added from a buret, and the volume added will be carefully observed. The following relationship will be used to calculate the concentration of the IO 3 - ion from the data: M IO 3 = ___ml S 2 O 3 2 ___M S 2 O 3 2 10.0 ml IO 3 1IO 3 6 S 2 O 3 2 (6) The number of ml of the S 2 O 3 2- and the M of the S 2 O 3 2- are the experimentally found values. The other numbers result from the conditions for this titration. INTRODUCTION For slightly soluble ionic compounds, equilibrium exists between a solid substance and its dissolved ions. For example, in a saturated solution of AgCl, about 0.002 grams of the substance dissolves in a liter of water. The equilibrium can be shown in the equation: AgCl(s) Ag + (aq) + Cl - (aq) (1) The equilibrium condition for this saturated solution is called the solubility constant, K sp , and has the form: K sp = [Ag + ][Cl - ] The solid AgCl is not used in the expression. If either the concentration of the Ag + or the Cl - is increased by the addi- tion of another ionic compound into the solution, the value of the K sp remains the same, but the solubility of the AgCl decreases. This would take place, for example, if the AgCl
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