Part ii test an unknown solution of scn 10 obtain

Part II Test an Unknown Solution of SCN –
10. 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.
11. Using the solution in the beaker, rinse a cuvette twice with ~1 mL amounts and then fill
it 3/4 full. Place the cuvette of unknown in the device (Colorimeter).
12. Determine the concentration of the unknown SCN
–
solution.
a. Tap the Meter icon located on the top left-hand corner.
b. Monitor the absorbance value. When this value has stabilized, record it in your data table.
c. Tap the Graph tab. On the Graph screen, choose Interpolate from the Analyze menu.
Tap any point on the regression cu rve (or use the ◄ or ► keys on LabQuest) to determine
the concentration of your unknown SCN – solution. Record the concentration in your data
table.
Part III Prepare and Test Equilibrium Systems
13. Prepare four test tubes of solutions, according to the chart below. Repeat Steps 11 and 12
from Part II to test the absorbance values of each mixture. Record the test results in your data
table. Record the [FeSCN
2+
] in the table in Data Analysis section #3. Note : You are using
0.0020 M Fe(NO
3
)
3
in this test.
Test tube
number
0.0020 M Fe(NO
3
)
3
(mL)
0.0020 M SCN
–
(mL)
H
2
O
(mL)
blank 3.00 0.00 7.00
1 3.00 2.00 5.00
2 3.00 3.00 4.00
3 3.00 4.00 3.00
4 3.00 5.00 2.00

The Determination of an Equilibrium Constant
Los Angeles City College
10 - 5
REPORT SHEET EXPERIMENT 10 NAME: ____________
PRE-LAB EXERCISE
For the solutions that you will prepare in Step 2 of Part I, 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. Show your calculations in your lab notebook.
Beaker number [FeSCN
2+
]
1
2
3
4
(blank) 0.00 M
POST-LAB REPORT
DATA TABLE FOR
PARTS I & II
Beaker Absorbance
[FeSCN
2+
]
1
2
3
4
Unknown, Part II
DATA TABLE FOR PART III
Test tube number Absorbance
1
2
3
4

Chemistry 102
10 - 6
Los Angeles City College
DATA ANALYSIS
1. (Part II) Use the calibration equation from Item 1 and the absorbance reading for your
unknown solution to determine [SCN – ].
2. (Part II) Compare your experimental [SCN
–
], of your unknown, with the actual [SCN
–
].
Suggest reasons for the disparity.
3. (Part III) Use the absorbance values, along with the best fit line equation of the standard
solutions in Part I to determine the [FeSCN
2+
] at equilibrium for each of the mixtures that
you prepared in Part III. Complete the table and give an example of your calculations
below.
Test tube number
1 2
[FeSCN
2+
]
4. (Part III) Calculate the equilibrium concentrations for Fe 3+ and SCN – for the mixtures in
Test Tubes 2 – 5 in Part III. Complete the table and give an example of your calculations.
Test tube number
1 2
[Fe
3+
]
[SCN – ]
5. Calculate the value of K
eq
for the reaction. Explain how you used the data to calculate
K
eq
.
3 4
3 4