b. Discard the cuvette contents as directed. Rinse the cuvette twice with the Test Tube 2 solution, fill the cuvette 3/4 full, and place it in the device. After the reading stabilizes, click , type 2 in the edit box, and click . c. Repeat the Step b procedure to find the absorbance of the solutions in Test Tubes 3, 4, and 5 (the standard solution). d. From the table, record the absorbance values for each of the five trials in your data table. e. Dispose of all solutions as directed by your instructor. PROCESSING THE DATA 1. Write the K c expression for the reaction in the Data and Calculation table. 2. Calculate the initial concentration of Fe 3+ , based on the dilution that results from adding KSCN solution and water to the original 0.0020 M Fe(NO 3 ) 3 solution. See Step 2 of the procedure for the volume of each substance used in Trials 1-4. Calculate [Fe 3+ ] i using the equation (will be the same for all four test tubes): [Fe 3+ ] i = Fe(NO 3 ) 3 mL total mL ! (0.0020 M) 3. Calculate the initial concentration of SCN – , based on its dilution by Fe(NO 3 ) 3 and water: [SCN – ] i = KSCN mL total mL ! (0.0020 M) In Test Tube 1, [SCN – ] i = (2 mL / 10 mL)(0.0020 M) = 0.00040 M. Calculate this for the other three test tubes. 4. [FeSCN 2+ ] eq is calculated using the formula: [FeSCN 2+ ] eq = A eq A std ! [FeSCN 2+ ] std where A eq and A std are the absorbance values for the equilibrium and standard test tubes, respectively, and [FeSCN 2+ ] std = (1/10)(0.0020) = 0.00020 M. Calculate [FeSCN 2+ ] eq for each of the four trials. 5. [Fe 3+ ] eq : Calculate the concentration of Fe 3+ at equilibrium for Trials 1-4 using the equation: [Fe 3+ ] eq = [Fe 3+ ] i – [FeSCN 2+ ] eq 6. [SCN – ] eq : Calculate the concentration of SCN - at equilibrium for Trials 1-4 using the equation: [SCN – ] eq = [SCN – ] i – [FeSCN 2+ ] eq 7. Calculate K c for Trials 1-4. Be sure to show the K c expression and the values substituted in for each of these calculations. 8. Using your four calculated K c values, determine an average value for K c . How constant were your K c values? Page 73 of 77
Chemical Equilibrium: Finding a Constant, K c DATA AND CALCULATIONS Absorbance Trial 1 _______ Trial 2 _______ Trial 3 _______ Trial 4 _______ Absorbance of standard (Trial 5) _______ Temperature _______ °C K c expression K c = [Fe 3+ ] i [SCN – ] i [FeSCN 2+ ] eq [Fe 3+ ] eq [SCN – ] eq K c value Average of K c values K c = ________ at ________°C Page 74 of 77
Lab 16 The Determination of the Percent Water in a Compound The polarity of the water molecule, which makes it a great solvent for ionic compounds, causes water molecules to cling to the structure of solid substances. When this occurs, the trapped water molecules are called water of hydration and they become an integral part of the crystal structure. There are many compounds that have a tendency to absorb water vapor from the air. These compounds are said to be hygroscopic , and can be used as moisture-reducing agents. Other compounds absorb such large quantities of water vapor that they will actually dissolve in their own water of hydration, a property known as deliquescence .
You've reached the end of your free preview.
Want to read all 77 pages?
- Fall '10