+ ] eq allows you to determine the concentrations of the other two ions at equilibrium. For each mole of FeSCN 2 + ions produced, one less mole of Fe 3 + ions will be found in the solution (see the 1:1 ratio of coefficients in the equation on the previous page). The [Fe 3 + ] can be determined by: [Fe 3 + ] eq = [Fe 3 + ] i – [FeSCN 2 + ] eq Because one mole of SCN - is used up for each mole of FeSCN 2 + ions produced, [SCN – ] eq can be determined by: [SCN – ] eq = [SCN – ] i – [FeSCN 2 + ] eq Knowing the values of [Fe 3 + ] eq , [SCN – ] eq , and [FeSCN 2 + ] eq , you can now calculate the value of K c , the equilibrium constant. MATERIALS Macintosh or Windows PC 0.0020 M KSCN Vernier computer interface 0.0020 M Fe(NO 3 ) 3 (in 1.0 M HNO 3 ) Logger Pro 0.200 M Fe(NO 3 ) 3 (in 1.0 M HNO 3 ) Vernier Colorimeter four pipets 1 plastic cuvette pipet bulb or pipet pump five 20 X 150 mm test tubes three 100-mL beakers thermometer tissues (preferably lint-free) PROCEDURE 1. Obtain and wear goggles. 2. Label four 20 X 150 mm test tubes 1-4. Pour about 30 mL of 0.0020 M Fe(NO 3 ) 3 into a clean, dry 100-mL beaker. Pipet 5.0 mL of this solution into each of the four labeled test tubes. Use a pipet pump or bulb to pipet all solutions. CAUTION: Fe(NO 3 ) 3 solutions in this experiment are prepared in 1.0 M HNO 3 and should be handled with care. Pour about 25 mL of the 0.0020 M KSCN into another clean, dry 100-mL beaker. Pipet 2, 3, 4 and 5 mL of this solution into Test Tubes 1-4, respectively. Obtain about 25 mL of distilled water in a 100-mL beaker. Then pipet 3, 2, 1 and 0 mL of distilled water into Test Tubes 1-4, respectively, to bring the total volume of each test tube to 10 mL. Mix each solution thoroughly with a stirring rod. Be sure to clean and dry the stirring rod after each mixing. Measure and record the temperature of one of the above solutions to use as the temperature for the equilibrium constant, K c . Volumes added to each test tube are summarized below: Test Tube Number Fe(NO 3 ) 3 (mL) KSCN (mL) H 2 O (mL) 1 5 2 3 2 5 3 2 3 5 4 1 4 5 5 0 3. Prepare a standard solution of FeSCN 2 + by pipetting 18 mL of 0.200 M Fe(NO 3 ) 3 into a 20 X 150 mm test tube labeled “5”. Pipet 2 mL of 0.0020 M KSCN into the same test tube. Stir thoroughly. 4. Prepare the computer for data collection by opening the file in the Experiment 20 folder of Chemistry with Computers . You should see a live Meter window to display absorbance, and a Table window with columns for the trial number and the absorbance value.
Chemical Equilibrium: Finding a Constant, K c CHM104_Exp#7_Equilibrium Constant Al-Ansari_Sp10 3 5. Prepare a
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