K sp = [Pb 2+ ][I ] 2 (1)

Transcription

1 Chem 1B Saddleback College Dr. White 1 Experiment 11: Determination of K sp Objectives To determine the concentration of an unknown using a Beer- Lambert Plot. To determine the K sp for a relatively insoluble salt Introduction In this experiment, you will determine the solubility product, Ksp, for a relatively insoluble compound, PbI 2. The equation for the solution of PbI 2 is: PbI 2(s) Pb 2+ (aq) + 2I - (aq) The solubility product expression associated with this reaction is K sp = [Pb 2+ ][I ] 2 (1) A saturated solution contains solid PbI 2 in equilibrium with its ions. The equilibrium system can be established by simply dissolving solid PbI 2 in distilled water until no more dissolves or by mixing solutions of Pb(NO 3) 2 and KI to form the precipitate which can then be removed from the solution. We will establish the equilibrium by mixing the ions to form a precipitate. Then we will remove the solid PbI 2 by using a centrifuge and a filter system. Then the [I - ] remaining in solution at equilibrium can be determined spectrophotometrically. Although the iodide ion is not colored, it is relatively easily oxidized to I 2, which is brown in water solution. We will oxidize the I - by reacting it with NO 2 - ions in an acidic environment where the conversion to I 2 is quantitative. Although the concentration of I 2 will be rather low in the solutions you will prepare, the absorption of light by I 2 in the vicinity of 456 nm is sufficiently intense to make accurate analyses possible. In all of the solutions prepared, 0.2 M potassium nitrate KNO 3 (note this distinction between KNO 2 and KNO 3!) will be present as an inert salt. This salt serves to keep the ionic strength of the solution essentially constant at 0.2 M and promotes the formation of well- defined crystalline precipitates of PbI 2. Like Experiment 1 (determination of an equilibrium constant), you will be using a Spectrometer connected to Logger Pro and creating a standard curve (Beer s Law Plot). Procedure Note: Dispose of all chemicals in the provided waste containers. DO NOT pour any chemicals down the drain. Use a large beaker and collect all your waste which you can then dump in the waste container at the end of the experiment. 1. Centrifuge solutions A- D (the equilibrium solutions you prepared last time) for 2 minutes at about 3000 rpms to settle any suspended PbI 2 solid. Then, get a clean, dry plastic syringe. Remove the plunger and place a small wad of cotton tightly into the bottom of the syringe. Decant the solution into the syringe with the syringe placed over a clean, dry test tube. Replace the plunger and very slowly push the plunger down to force the liquid into the test tube. Discard the syringe in the collection container. The solution should now be free of the precipitate. Repeat the procedure above for each solution in test tubes B- D. Use a new syringe for each tube. 2. Pipet 3.0 ml of the clear, colorless solutions found in test tubes A- D into other dry tubes (label A - D ), then to each tube add 3.0 ml of M KNO 2 (aq), and two drops of 6 M HCl (aq) to oxidize the I - to I 2. Wait for 10 minutes for the color to develop. 3. Calibrate the spectrometer (you should know how to do this by now. If not, refer to Exp. 10) 4. Empty the blank cuvette and rinse it twice with small amounts of the solution from test tube A. Fill the cuvette ¾ full with the test tube A solution and place it in the

2 Chem 1B Saddleback College Dr. White 2 spectrometer. Click. Click to complete the analysis. Record the absorbance at at the λ max determined previously. 5. Repeat step 4 for the solutions in test tubes B - D. 6. Use the calibration curve from the Beer s Law plot to determine the [I - ] at equilibrium in each tube. Use an ICE chart to record the initial [Pb 2+ ] and [I - ], the changes that occurred, and the equilibrium [Pb 2+ ]. Use these equilibrium concentrations to calculate the K sp value for each tube. Dispose of all Pb solids and solutions into the heavy metal waste container.

3 Chem 1B Saddleback College Dr. White 2 Name: Lab Day/Time: Data and Results: Experiment 11 Determination of K sp The PbI 2 (s) Pb 2+ (aq) + 2I - (aq) Equilibrium System Initial Concentrations of Pb 2+ and I - : Tube # [Pb 2+ ] initial [I - ] initial (mol/l) (mol/l) Show calculations for how you obtained the Pb 2+ and I - initial concentrations in Tube 2. Equilibrium Concentrations of Pb 2+ and I - : Tube # Absorbance [I - ] equil [Pb 2+ ] equil K sp Average K sp How is [I - ] equil obtained? Show your calculation for Tube 2.

4 Chem 1B Saddleback College Dr. White 3 How is [Pb 2+ ] equil obtained? Show your calculations for Tube 2. Show how you determined the K sp value for Tube 2:

5 Chem 1B Saddleback College Dr. White 4 Post-Lab Questions 1. When 5.00 ml of M Pb(NO 3) 2 are mixed with 5.00 ml of M KI, a yellow precipitate of PbI 2(s) forms. In a spectrophotometer the equilibrium solution is analyzed for I -, and its absorbance is found to be a. Using the Beer- Lambert Plot and line of best fit from the post- lab questions in Experiment 10, determine the concentration of iodide at equilibrium. [I - eqilibrium] = M b. Calculate the initial concentrations of the Pb 2+ and I - ions. [Pb 2+ ] initial M: [I- ] initial M c. Use the initial concentrations of the Pb 2+ and I - ions along with the equilibrium concentration of the I - ion and the reaction stoichiometry to determine the equilibrium concentration of the Pb 2+ ion. d. What is the value of K sp for the reaction? [Pb 2+ ] equil M

6 Chem 1B Saddleback College Dr. White 5 2. A solution is prepared by mixing 20.0 ml of M Sc 3+ with 40.0 ml of M OH -. After mixing, a precipitate of Sc(OH) 3 settles out, and the resulting solution is found spectrophotometrically to be M in Sc 3+. Determine the K sp of Sc(OH) 3.