C choose calibrate ch1 colorimeter from the

c. Choose Calibrate

CH1
: Colorimeter from the Experiment menu, then click .
d. Turn the wavelength knob on the Colorimeter to the “0% T” position.
e. Type “0” in the edit box.
f. When the displayed voltage reading for Reading 1 stabilizes, click .
g. Turn the knob of the Colorimeter to the Blue LED position (470 nm).
h. Type “100” in the edit box.
i. When the voltage reading for Reading 2 stabilizes, click , then click .
6. You are now ready to collect absorbance data for the standard solutions. Click to
begin data collection. Note : Take readings within 4 minutes of preparing the mixtures.
a. Empty the water from the cuvette. Using the solution in Beaker 1, rinse the cuvette twice
with ~1 mL amounts and then fill it ¾ full. Wipe the outside with a tissue, place it in the
Colorimeter, and close the lid. Wait for the absorbance value displayed in the Meter
window to stabilize. Click , type the concentration of FeSCN
2+
(from your pre-lab
calculations) in the edit box, and press the
ENTER
key.
b. Discard the cuvette contents as directed. Rinse and fill the cuvette with the solution in
Beaker 2. Follow the procedure in Part a of this step to measure the absorbance, and enter
the concentration of this solution.
c. Repeat Part b of this step to measure the absorbance of the solutions in Beakers 3, 4,
and 5.
d. Click when you have finished collecting data. Click the Examine button, , and
record the absorbance values for each data pair.

The Determination of an Equilibrium Constant
7. Click the Linear Fit button, . A best-fit linear regression line will be shown for your five
data points. This line should pass near or through the data points and the origin of the graph.
(Note: Another option is to choose Curve Fit from the Analyze menu, and then select
Proportional. The Proportional fit has a y-intercept value equal to 0; therefore, this regression
line will always pass through the origin of the graph).
Leave the graph and best fit line displayed and proceed to Step 8.
Part II Test an Unknown Solution of SCN
–
8. Obtain about 10 mL of the unknown SCN
–
solution. Use a pipet to measure out 5.0 mL of the
unknown into a clean and dry 100 mL beaker. Add precisely 5.0 mL of 0.200 M Fe(NO
3
)
3
and 40.0 mL of distilled water to the beaker. Stir the mixture thoroughly.
9. Using the solution in the beaker, rinse a cuvette twice with ~1 mL amounts and then fill
it ¾ full. Wipe the outside with a tissue, place it in the Colorimeter, and close the lid.
Watch the absorbance readings in the Meter window. When the readings stabilize, record the
absorbance value for your unknown in your data table. Remove and clean the cuvette.
10. Determine the concentration of the unknown SCN
–
solution.
a.