CHM 2046 lab manual 8.9 (21) 8

CHM2046 Equilibrium Constant Determination St. Petersburg College Experimental Procedure Materials x 5 clean and dry test tubes x 2.00 x 10 -3 M Fe(NO 3 ) 3 in 1 M HNO 3 solution x 2.00 x 10 -3 M KSCN solution x Cuvettes and UV-Visible Sprectrophotometer 1. Obtain 5 clean and dry medium test tubes. Label them 1, 2, 3, 4, and 5. 2. Obtain two clean and dry small beakers. 3. Pour ~30 mL of 2.00x10 -3 M Fe(NO 3 ) 3 (this comes dissolved in 1M HNO 3 , so be very careful! Nitric acid is extremely corrosive and toxic. Use gloves!) into the first beaker. 4. Pour ~20 mL of 2.00 x10 -3 M KSCN into the second beaker. 5. Measure out the beaker solutions and deionized water into each test tube according to Table 1. The total solution in each test tube will be 10.00 mL each. Use a clean graduated cylinder for each solution and for deionized water to avoid cross- contamination. Table 1. Test tube mixture ratios, measured absorbance at 447 nm and calculated concentration of FeSCN 2+ . 6. After the solutions are made in the test tubes, stir/mix well so a homogeneous solution is made. If a glass rod is used, be sure to clean and dry the glass rod before reusing it in the next test tube. 7. Leave the test tubes on your test tube rack. Obtain a plastic pipet for each test tube. 8. Obtain a clean and dry cuvette from the instructor, or from inside the spectrophotometer. Remember, the cuvettes may look like small test tubes but they are different from test tubes. 25

CHM2046 Equilibrium Constant Determination St. Petersburg College Recorded Data Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Absorbance Reading Calculated Data Step 1: Calculate the initial moles of Fe 3+ and SCN - in the solutions. Equation 3 ܫ݊݅ݐ݈݅ܽ ݉݋݈݁ݏ ܨ݁ ଷା = 2.00 × 10 ିଷ ܯ ܨ݁ ଷା × ܸ݋݈ݑ݉݁ ܷݏ݁݀ ( ݉ܮ ) × ଵ ௅ ଵ଴଴଴ ௠௅ Equation 4 ܫ݊݅ݐ݈݅ܽ ݉݋݈݁ݏ ܵܥܰ ି = 2.00 × 10 ିଷ ܯ ܵܥܰ ି × ܸ݋݈ݑ݉݁ ܷݏ݁݀ ( ݉ܮ ) × ଵ ௅ ଵ଴଴଴ ௠௅ Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Initial moles Fe 3+ Initial moles SCN - Step 2: Calculate the equilibrium concentration of FeSCN 2+ and equilibrium moles of FeSCN 2+ Equation 5 ܧݍݑ݈ܾ݅݅ݎ݅ݑ݉ [ ܨ݁ܵܥܰ ଶା ]( ݅݊ ௠௢௟ ௅ ) = ௔௕௦௢௥௕௔௡௖௘ ସ଻ଵଷ . ଺ ௅ / ௠௢௟ Equation 6 ܧݍ ᇱ ݉ ݉݋݈݁ݏ ܨ݁ܵܥܰ ଶା = ܧݍ ᇱ ݉ [ ܨ݁ܵܥܰ ଶା ] × 10.00 ݉ܮ × ଵ ௅ ଵ଴଴଴ ௠௅ Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Equilibrium [FeSCN 2+ ] Equilibrium moles FeSCN 2+ Step 3: Calculate the equilibrium moles of Fe 3+ and SCN - Equation 7 ܧݍ ᇱ ݉ ݉݋݈݁ݏ ܨ݁ ଷା = ܫ݊݅ݐ݈݅ܽ ݉݋݈݁ݏ ܨ݁ ଷା െ ܧݍ ᇱ ݉ ݉݋݈݁ݏ ܨ݁ܵܥܰ ଶା Equation 8 ܧݍ ᇱ ݉ ݉݋݈݁ݏ ܵܥܰ ି = ܫ݊݅ݐ݈݅ܽ ݉݋݈݁ݏ ܵܥܰ ି െ ܧݍ ᇱ ݉ ݉݋݈݁ݏ ܨ݁ܵܥܰ ଶା Test Tube 1 Test Tube 2 Test Tube 3 Test Tube 4 Test Tube 5 Equilibrium moles Fe 3+ Equilibrium moles SCN - 29 0.24 0.42 0.60 0.78 0.94