1. Colorimetric Determination of Keq
Experiment 22
Colorimetric Determination of Keq
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2. In Lab
Work in groups of 4
2 people do part A; preparing the standard solutions
2 people do part B; preparing the solutions at equilibrium
Everyone makes their own calibration curve
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3. Part A Standards for the Calibration Curve
Reation is “forced” to the product side, assume no equilibrium
Prepare all solutions at the same time
Use correct Fe solution (2.00x10-1 M)
Flask 1 (solution 1) is blank (no SCN)
Use 0.10 M HNO3 to prepare solns., not water
Use disposable test tubes instead of cuvettes for use in the spectrophotometer
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4. Part B Preparation of Equilibrium Solutions
Prepare all solutions at the same time
Use the correct Fe solution (2.00x10-3 M)
Prepare a blank solution (5.00 mL Fe, no SCN, no H2O)
Use 0.10 M HNO3 to prepare solns., not water.
Use disposable test tubes instead of cuvettes for use in the spectrophotometer
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5. Spectrophotometer (Parts A and B)
An instrument that will indicate the amount of light (of one wavelength) absorbed by a sample
readout
detector
sample
monochromator
Light source
slit
LED readout
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6. Making the Calibration Curve
You will acquire an absorbance reading for each standard solution prepared (share data)
Use the data from Part A to construct the calibration curve
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7. Calibration Curve
Absorbance
Concentration [FeNCS2+] x 10-5 M
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8. Using the Calibration Curve to Find Equilibrium Concentration of FeNCS2+
Absorbance
Concentration [FeNCS2+] x 10-5 M
Part B solution absorbance reading
Concentration of solution
Show this on your graph
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9. For All Students
There are two different Fe(NO3)3 solutions; Part A uses a 2.00x10-1M solution, and Part B uses a x10-3M solution.
The 0.10M HNO3 solution is added to test tubes 1 – 5, not 1 – 6 in part B (page 289), to get to a total volume of 10 mL in each test tube.
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