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Unformatted text preview: Experiment 26: Choice 2: An Equilibrium Constant
Pre-laboratory Questions: Show calculations where applicable for credit.
(use these and not the ones in the Experimental Chemistry Lab Manual)
1. The many colors of autumn leaves, the shades of colors in fingernail polishes and facial makeup, and the
colors of paints and dyes…all colors appear in the visible part of the electromagnetic spectrum. Explain
the mechanism by which molecules for each of these examples produce the unique colors that our eyes
2. The white light from a lamp of a spectrophotometer passes through a slit and focusing lens before
reflecting off a diffraction grating. The diffraction grating acts much like a prism, separating the white light
into a rainbow of colors. In this experiment, the wavelength control of the spectrophotometer is set to
maximize the absorption of 447-nm EM radiation for FeNCS2+ in the sample cell.
a. What is the color of 447-nm EM radiation?
b. What is the color of the FeNCS2+ sample in the experiment?
c. Why do the colors not agree?
3. When white light is passed through a colored solution, EM radiation is absorbed and EM radiation is
a. What is the quantitative relationship between absorbed and transmitted EM radiation?
b. Write the equation that relates absorbed EM radiation to the concentration of the absorbing species in
4. What is meant by a calibration curve for a colored substance being studied with a spectrophotometer?
How is a calibration curve used to determine the concentration of a solution on unknown concentration of
the colored substance?
5. Plot the following data as absorbance versus [X] as a calibration curve (USE EXCEL, ADD TRENDLINE,
AND ADD THE REGRESSION LINE EQUATION TO YOUR GRAPH):
Standard Concentration of X
3.0 x 10-4 mol/L
6.2 x 10-4 mol/L
9.0 x 10-4 mol/L
2.2 x 10-3 mol/L
3.2 x 10-3 mol/L
A “test” solution showed a percent transmittance (%T) reading of 55%T. Interpret the calibration curve to
determine the molar concentration of X in the test solution.
6. A reaction mixture of 2.0mL of 0.002 M SCN- and 5.0mL of 0.002 M Fe3+ is diluted to 10.0 mL with
deionized water. The molar concentration of the FeNCS2+ that forms at equilibrium, determined from a
calibration curve, is 1.2 x 10-4 mol/L. Calculate, in sequence, each of the following quantities in the
aqueous solution to determine the equilibrium constant for the reaction. Label and show work below this
Fe3+(aq) + SCN-(aq) ↔ FeNCS2+(aq)
a. moles of FeNCS that form in reaching equilibrium
b. moles of Fe3+ that react to form the FeNCS2+ at equilibrium
c. moles of SCN- that react to form the FeNCS2+ at equilibrium
d. moles of Fe3+ initially placed in the reaction system
e. moles of SCN- initially placed in the reaction system
f. moles of Fe3+ that remain unreacted at equilibrium (d-b)
g. moles of SCN- that remain unreacted at equilibrium (e-c)
h. molar concentration of Fe3+ (unreacted) at equilibrium
j. molar concentration of FeNCS2+ at equilibrium
_1.2 x 10-4 mol/L_
k. Kc = [FeNCS2+]/[Fe3+][SCN-] Experiment 26: An Equilibrium Constant
Post Lab Questions (use these and not the ones in the Experimental Chemistry Lab Manual):
1. All spectrophotometers are different. An experiment requires the spectrophotometer to be set at 447 nm.
What experiment could you do, what data would you collect, and how would you analyze the data to ensure
that 447 nm is the “best” setting for measuring the absorbance of FeNCS2+ in this experiment?
2. Fingerprints are not removed from the cuvet of a sample solution.
a. How does this affect the absorbance reading of the spectrophotometer?
b. Will the [Fe3+] equilibrium constant be determined to be too high or too low? Explain.
c. Will the calculated Kc be too high or too low? Explain.
3. Consider the table below with the following set of equilibrium test solutions for the determination of K using
measurements from a spectrophotometer. What is the purpose of adding the varying amounts of 0.1 M
HNO3, depending on the sample solution?
5 .0002M Fe(NO3)3
(in 0.1M HNO3)
5 0.002M NaSCN
(in 0.1M HNO3)
5 0.1M HNO3
0 4. The Beer’s Law Equation, A = c ε l , becomes nonlinear at high concentrations of the absorbing substance.
Note: c = concentration, ε = absorptivity constant, l = length of cuvette (or length of the width of the cuvette
container through which light passes). Suppose you prepare a solution with a very high absorbance that is
suspect in not following the linear relationship. How might you still use the sample for your analysis, rather
than discarding the sample and the data?
5. An analysis of a series of repeated measurements produced an equilibrium constant of 4.76 with a standard
deviation of 0.41. Explicitly state the analytical interpretation of this standard deviation with respect to the
measured equilibrium constant.
6. Glass cuvets, used for precision absorbance measurements in a spectrophotometer, are marked so that
they always have the same orientation in the sample compartment. What error does this minimize and why? ...
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This note was uploaded on 12/09/2010 for the course AS 124 taught by Professor Asdd during the Spring '10 term at Orange Coast College.
- Spring '10