Colorimetric Determination of an Equilibrium Constant in Aqueous Solution
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Colorimetric Determination of an Equilibrium Constant in Aqueous Solution
Purpose
The purpose of this lab is to understand the concept of equilibrium by determining the equilibrium constant for a reaction in solution.
Equipment/Materials
- Six 25.0 mL volumetric flasks
- Six Large test tubes
- Solutions: 60mL of 0.20 M Fe(NO3)3 (made with HNO3), 22.5 mL of 0.0020 M NaSCN, 82.5 mL of DI H2O, 30 mL of 0.0020 M Fe(NO3)3, and 15 mL of 0.10 M HNO3
- Spectrophotometer
Procedure
We began by preparing the solutions to determine the calibration curve given in this table:
Solution | Volume of 0.20 M Fe(NO3)3 (made with HNO3) | Volume of 0.0020 M NaSCN | Volume of DI H2O | Total Volume |
1 (blank) | 10.00 mL | 0.0 mL | 15.00 mL | 25.00 mL |
2 | 10.00 mL | 0.50 mL | 14.50 mL | 25.00 mL |
3 | 10.00 mL | 1.00 mL | 14.00 mL | 25.00 mL |
4 | 10.00 mL | 1.50 mL | 13.50 mL | 25.00 mL |
5 | 10.00 mL | 2.00 mL | 13.00 mL | 25.00 mL |
6 | 10.00 mL | 2.50 mL | 12.50 mL | 25.00 mL |
Once each solution was made, we measured the transmittance of each solution at 447 nm using the spectrophotometer. We then recorded the values. Subsequently, we prepared the solutions to determine the equilibrium constant given in this table:
Solution | Volume of 0.0020 M Fe(NO3)3 | Volume of 0.0020 M NaSCN | Volume of 0.10 M HNO3 | Total Volume |
1 (blank) | 5.00 mL | 0.00 mL | 5.00 mL | 10.00 mL |
2 | 5.00 mL | 1.00 mL | 4.00 mL | 10.00 mL |
3 | 5.00 mL | 2.00 mL | 3.00 mL | 10.00 mL |
4 | 5.00 mL | 3.00 mL | 2.00 mL | 10.00 mL |
5 | 5.00 mL | 4.00 mL | 1.00 mL | 10.00 mL |
6 | 5.00 mL | 5.00 mL | 0.00 mL | 10.00 mL |
Once each solution was made, we measured the transmittance of each solution at 447 nm using the spectrophotometer. We then recorded the values.
Results
Solution | Transmittance |
1 (blank) | 100 |
2 | 70 |
3 | 44 |
4 | 29 |
5 | 20 |
6 | 13 |
Table 1) Transmittance values from spectrophotometry for calibration curve
Table 2) Transmittance values from spectrophotometry for equilibrium constant
Solution | Transmittance |
1 (blank) | 100 |
2 | 80 |
3 | 62 |
4 | 48 |
5 | 37 |
6 | 30 |
Table 3) Calculations for Beer’s Law Plot
%T | T (%T/100) | 1/T | A (log10(1/T)) | [FeSCN2+] * |
70 | 0.7 | 1.428571429 | 0.15490196 | 0.00004 |
44 | 0.44 | 2.272727273 | 0.356547324 | 0.00008 |
29 | 0.29 | 3.448275862 | 0.537602002 | 0.00012 |
20 | 0.2 | 5 | 0.698970004 | 0.00016 |
13 | 0.13 | 7.692307692 | 0.886056648 | 0.0002 |
*See calculations to see how concentration of FeSCN2+ is determined
[pic 1]
Figure 1) Graph depicts the relationship between molar concentration of FeSCN2+ and the calculated absorbance we obtained from experimental transmittance. The equation located within the graph provides the relationship between absorbance and concentration that is determined by Beer’s Law. This relationship is shown by the extinction coefficient (4412.3 cm*L/mol).
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