Test solutions of scn of unknown molar concentration


Test solutions of SCN – of unknown molar concentration.

Determine the molar concentrations of the ions present in an equilibrium system.

Determine the value of the equilibrium constant, K
eq
, for the reaction.

10 - 2
Advanced Chemistry with Vernier
LabQuest 10
MATERIALS
Vernier LabQuest
LabQuest App
0.200 M iron (III) nitrate, Fe(NO
3
)
3
,
solution in 1.0 M HNO
3
Vernier Colorimeter or Spectrometer
plastic cuvette
0.0020 M iron (III) nitrate, Fe(NO
3
)
3
,
solution in 1.0 M HNO
3
four 10.0 mL pipettes
pipet pump or bulb
potassium thiocyanate, KSCN solution
of unknown concentration
six 20 × 150 mm test tubes 0.0020 M thiocyanate, SCN
–
test tube rack distilled water
eight 100 mL beakers tissue
50 mL volumetric flask Temperature Probe (optional)
plastic Beral pipets
PRE-LAB EXERCISE
For the solutions that you will prepare in Step 2 of Part I below, calculate the [FeSCN 2+ ].
Presume that all of the SCN – ions react. In Part I of the experiment, mol of SCN – = mol of
FeSCN 2+ . Thus, the calculation of [FeSCN 2+ ] is: mol FeSCN 2+ ÷ L of total solution. Record these
values in the table below.
Beaker number [FeSCN 2+ ]
1
2
3
4
(blank) 0.00 M
PROCEDURE
Part I Prepare and Test Standard Solutions
1. Obtain and wear goggles.
2. Label five 100 mL beakers 1–5. Obtain small volumes of 0.200 M Fe(NO
3
)
3
,
0.0020 M SCN – , and distilled water. CAUTION: Fe(NO
3
)
3
solutions in this experiment are
prepared in 1.0 M HNO
3
and should be handled with care. Prepare four solutions according
to the chart below (The fifth beaker is a blank.). Use a 10.0 mL pipet and a pipet pump or
bulb to transfer each solution to a 50 mL volumetric flask. Mix each solution thoroughly.
Measure and record the temperature of one of the above solutions to use as the temperature
for the equilibrium constant, K
eq
.
Beaker
number
0.200 M Fe(NO
3
)
3
(mL)
0.0020 M SCN –
(mL)
H
2
O
(mL)
1 5.0 2.0 43.0
2 5.0 3.0 42.0
3 5.0 4.0 41.0
4 5.0 5.0 40.0
blank 5.0 0.0 45.0

Advanced Chemistry with Vernier
10 - 3
The Determination of an Equilibrium Constant
Note : The fifth beaker is prepared to be used as a blank for your spectrometer (or Colorimeter)
calibration. It will have a slightly yellow color due to the presence of Fe(NO
3
)
3
. By calibrating
with this solution as your blank, instead of distilled water, you will account for this slight yellow
color.
3. Prepare a blank by filling a cuvette 3/4 full of the solution in the fifth beaker. To correctly use
cuvettes, remember:

Wipe the outside of each cuvette with a lint-free tissue.

Handle cuvettes only by the top edge of the ribbed sides.

Dislodge any bubbles by gently tapping the cuvette on a hard surface.

Always position the cuvette so the light passes through the clear sides.
Spectrometer Users Only
4. Connect the Spectrometer to LabQuest and choose New from the File menu.
5. Calibrate the Spectrometer.
a. Place the blank cuvette in the Spectrometer.
b. Choose Calibrate from the Sensors menu. The following message is displayed: “Waiting
60 seconds for lamp to warm up.” After 60 seconds, the message will change to “Warmup
complete.”
c. Select Finish Calibration. When the message “Calibration completed” appears, select OK.
6. Determine the optimum wavelength for the standard solution in Beaker 1 and set up the
data-collection mode.