Stoichiometry of a Metal Complex
The purpose of this experiment was to determine the mole ratio of , ortho-phenanthroline and the formula of this Fe(II) metal complex.
At 510 nm of a maximum absorbance, this experiment pursues the fact that the metal complex produced highly absorbs visible light. It is so obvious that the o-phen (ortho-phenanthroline) molecule and the aqueous ferrous ion absorb a very small amount of light at that lambda (510nm). The absorption of light by metal complex (A) is proportional to the molar concentration of that complex(C) from Beer’s law . In which () is the molar absorptivity and (b) is the pathlength are stable.
Procedure
A. Preparation of the Stock Fe(II) Solutions:
The solid was set up and used to make stock A. A 0.2703g of the solid was weighted and put into a 100mL volumetric flask that was cleaned. A 20mL of distilled water was added to the mark in the flask. The solution was diluted with distilled water. The flask was closed with a stopper, flipped upside down and gotten back to the correct way for few times until the solution was mixed. The solution was decanted into a clean and dry beaker. The beaker was marked Stock A Fe(II) solution. The molarity of Stock A was calculated. Stock A was used to do Stock B Fe(II) solution. A 25mL pipet was used, after it was washed with small portions of stock A solution, to pour 25mL of stock A solution into a clean 250mL volumetric flask.
4.9997g of hydroxylamine sulfate and 4.9967 of sodium sulfate were weighted and added tardily into the 250mL flask. The solid was dissolved by spinning the solution. A small amount of distilled water was added into the solution to dissolve the solid. The solution was diluted to the mark with distilled water. The flask was closed with stopper. The solution was mixed and poured into a clean and dry beaker that was labeled Stock B Fe(II) solution. The molarity of stock B was calculated.
B. Preparation of solution #1 and Calibration of the Spectrophotometer:
A 10mL pipet was washed with few small portions of stock B solution to make solution #1. A 10mL of Stock B solution was poured by using the 10mL pipet into a clean 100mL volumetric flask. The solution was diluted to the mark, mixed, and decanted into a clean dry beaker. The LabQuest was turned on and left to do some routing and testing. The directions 3-5 on page 43 of the lab manual were done and the wavelength was changed to 510nm. The cuvette was washed and filled with solution #1 and put into the Spectro. The green arrow was clicked and the calibrate screen was showed. The LabQuest was left to warm up for 90 s. The cuvette was removed and drained.
C. Preparation of Solutions 2-8 and measurement of absorbance:
The stock ortho-phenanthroline was set up and the concentration was recorded. The flask was washed with distilled water and the solutions 2-8 were prepared. A 10mL of Stock B solution was poured into a 100mL volumetric flask by using the 10mL pipet. About 1.94mL of o-phen was added into the flask and the Vi and Vf were recorded from the buret. The flask was filled to the mark with distilled water and stoppered. The solution was mixed and decanted into a clean dry beaker. The cuvette was washed with small amount of the solution, filled with it and put into the Spectro. The absorbance was measured and recorded. The steps above were duplicated for all solutions between 3-8 and the measurements were recorded in the lab notebook.
Data
Grams of ferrous ammonium sulfate
0.2703g
Grams of hydroxylamine sulfate
4.9997g
Grams of sodium sulfate
4.9967g
Molarity of ortho-phenanthroline
0.00242M
Sample #
mL Fe(II) Stock B (10.00 mL pipet)
Approximate mL of o-phen stock placed in the 100.0 mL flask
Vi
Buret
mL
Vf
Buret
mL
Actual o-phen used (Vi – Vf)
mL
ABS
1
10.00mL
0
– – –
– – –
0.00
Set to 0
2
10.00mL
~ 2
1.62
3.56
1.94
0.160
3
10.00mL
~ 4
3.56
7.38
3.82
0.327
4
10.00mL
~ 6
7.38
13.09
5.71
0.499
5
10.00mL
~ 8
13.09
20.87
7.78
0.690
6
10.00mL
~ 12
20.87
32.58
11.71
0.774
7
10.00mL
~ 15
32.58
47.39
14.81
0.771
8
10.00mL
≤ 20
28.24
47.98
19.74
0.772
Calculations
Molarities of Stock A & Stock B:
m-mole o-phen:
Sample #
Calculations
1
0.00
2
3
0.00925 m-mol
4
0.0138 m-mol
5
0.0188 m-mol
6
0.0283 m-mol
7
0.0358 m-mol
8
0.0478 m-mol
Average of “Horizontal” absorbance points:
X coordinate for the intersection point of the increasing & horizontal lines:
m-mole of Fe2+ in solution:
Calc. of “n”:
Graph
Discussion
The state result of this experiment was the absorbance in the second solution was low (0.160), but the absorbance became higher and higher (increased) on each time the solution got more concentrate by o-phenanthroline and the last absorbance (solution #8), it stopped in 0.772 and had no more rise. In the last 3 solutions, the absorbance was constant between 0.774, 0.771 and .0772; thus, it did not get more than 0.780. The thing that was not expected that after the absorbance got increased from 0.160 to 0.772, it will not get higher more than 0.780. The liner equation for the absorbance vs. mmole of o-phen was and the increasing absorbance region was in the slope = 37.5, and in the intersection = -0.018. The average horizontal absorbance region was 0.772. The x coordinate for the intersection point of the increasing & horizontal lines was 0.0212.
Source of errors:
Some of the errors in this experiment is incorrect measurements. For example, taking the initial or the final volume from the buret wrongly can give incorrect measurements for the mL of the o-phen solution. Another source of error is unclean equipment. Sometime unclean equipment would cause unclear observation and wrong measurements. Another source of error is wrong set for the LabQuest. For instance, set up the LabQuest in the incorrect way such as, not changing the wavelength, not changing the mode or not giving enough time to warm up can cause some incorrect measurements to the absorbance.
