March 16, 2011
Procedure A: Determination of the Equilibrium Constant
*Wear safety glasses at all times
*Never pipet a solution directly out of the bottle because it is likely to become contaminated. Instead pour some of the solution into a beaker beforepipeting.
*There are two different concentrations of Fe(NO3)3 used in this experiment so you must read the label of the bottle carefully to be sure it is the right one
a) Preparing the standard solutions
1. Measure the absorbances of all the standard solutions on the same spec
2. Measure the absorbances of your own equilibrium mix-tures
3. Prepare a standard solution from the table shown below
4. Dispense the amount of HSCN shown in the table from the dispensing buret into a 100 mL volumetric flask and dilute to the mark with 0.200 M Fe(NO3)3 in 0.500 M HNO3 solution
a. Make sure it is not 0.002 M
b. Check TECH Section II.G for instructions*Don’t put acetone into a cuvet as it will cloud the plastic
*You must use the same spec for all absorbance readings
5. Set the wavelength of the spec to 447 nm.
6. “Zero” the spec using the 0.200 M Fe(NO3)3 in 0.500 M HNO3 solution as a blank
7. Determine the absorbance of your assigned standard solution
8. Use thestandard solutions prepared by other groups to obtain absorbance measurements for all of the standard solutions using your group’s spec.
Solution | Number of mL of 0.00200 M HSCN in 0.500 M HNO3 |
A | 1.00 |
B | 2.00 |
C | 3.00 |
D | 4.00 |
E |5.00 |
F | 6.00 |
b) Preparing the equilibrium solutions
*This part is on your own
1. Label three clean, dry 18 x 150 mm test tubes as "2", "3" and "5"
2. Obtain about 35 mL of 0.00200 M Fe(NO3)3 in 0.500 M HNO3 solution in a clean, dry 250 mL beaker.
a. Make sure it is not 0.200MLabel the beaker
3. Rinse a 5.00 mL volumetric pipet with small portions of this solution.
4. Use this pipet to add 5.00 mL of this solution to each test tube.
5. Using the dispensing buret, dispense into each test tube 0.00200 M HSCN in 0.500 M HNO3 solution
6. Deliver the exact number of milliliters (2, 3 or 5) indicated on the labelof the test tube
7. Record the volume delivered.
8. Obtain about 15 mL of 0.500 M HNO3 solution in a clean, dry 150 mL beaker.
9. Label the beaker.
10. Rinse a measuring pipet with this solution
11. Add enough 0.500 M HNO3 to each test tube to make the total volume equal to 10.0 mL
b. 3, 2 or 0 mL,depending on the volume of 0.00200 M HSCN in 0.500 M HNO3 solution which was added
*When using a measuring pipet, the liquid is not drained to the tip and is stopped when the desired calibration mark is reached. The delivered volume is taken as the final reading minus the initial reading.
12. Mix each solution thoroughly with a glass stirring rod
c.Dry off the stirring rod after mixing each solution.
*Immediately proceed with the next step to avoid interfering side reactions
*Use the same spec that you used to measure the standard solutions
13. “Re-zero” the instrument using the 0.00200 M Fe(NO3)3 in 0.500 M HNO3 solution as a blank
14. Immediately, measure the absorbance of each...