Zach Bensley
Experiment 2
Gravimetric Determination of Iron as Fe2O3
Introduction:
One will determine the amount of iron in an unknown sample by precipitating the hydrated
iron oxide from a basic solution. The following equation should be completed prior to experiment:
Base
Fe3+ + 3H2O -----------> FeOOH * H2O (s) + 3H+
900 0C
2FeOOH * 2 H2O ------------> Fe2O3 + 2H2O(s)
Procedure:
Three porcelain crucibles and caps were fired until a constant weight was determined. The
crucible and lids were heated over a bunsen burner in the hood for 10 min. After the heating of the
crucible and lids, the crucible and lids were cooled for 15 min. This procedure was repeated until the
crucible and lid’s mass were reproducible.
Three samples of unknown iron product were massed. The mass of each iron product was
approximately 0.13 g. This was determined through further calculations. Each sample of iron was
dissolved in 3 M HCl. 5 mL of 6 M HNO3 was added into the solution and was boiled for 2 min.
The sample was the diluted with 200 mL of distilled water and 3 M of ammonia was added
until the solution was basic. The precipitate was then boiled for 5 min.
The supernatant was then decanted through ashless filter paper. The solid was washed with
1 % NH4NO3 until no Cl- was detected. The filter paper was then dried for 24 h, protected from dust.
Before placing the ashless filter paper with our precipitate on it, the oven used was set to
900 0C. The sample was dried and the filter paper was ignited using the crucibles and lids. The paper
was ignited slowly and the flames quenched immediately. When there was a flame, the lid was
automatically placed on top of the crucible. When the oven was open to put the crucibles in, the
temperature decreased. When the oven reached 900 0C, the precipitate was heated for 30 min. After
30 min, the precipitate was cooled for 20 min. After the cooling, the precipitate was massed and
recorded.
Data:
Mass of Crucibles and Lids:
Crucible and Lid
Trial 1 Mass (g)
1
49.8754
2
48.1156
3
45.4402
Mass of Unknown:
Sample of Unknown
A
B
C
Mass of Precipitate:
Crucible and Lid
1
2
Trial 2 Mass (g)
49.8754
48.1131
45.4376
Trial 3 Mass (g)
N/A
48.1131
45.4376
Mass of Unknown (g)
0.1398
0.1366
0.1398
Sample of Unknown
A
B
Mass of Crucible, lid
and precipitate (g)
49.8889
48.1267
Mass of Precipitate (g)
0.0135
0.0136
3
C
45.4500
0.0125
Mass of Solid and Final Precipitate:
Trial
1
2
3
Initial Solid Mass (g)
0.1398
0.1366
0.1398
Precipitate Mass (g)
0.0135
0.0136
0.0125
Calculated Data:
Theoretical Yield (g)
Percent Yield
Percent Recovery
Trial 1
0.0314
57.2%
42.8%
Trial 2
0.0307
55.7%
44.3%
Calculations:
Mass of Precipitate:
Mass of Precipitate:
Mass final-Mass initial= Mass of Precipitate
Mass final – Mass initial
49.8887 g – 49.8754 g = 0.0135 g
Mass final= Mass of Crucible, lid and solid
Mass initial= Mass of crucible and lid
Theoretical Yield
M initalsolid * 0.75 *

1molFeNH4 (SO4 ) 2
1molFe
1molFe2 0 3 159.69gFe2 0 3
*
*
265.937gFeNH4 (SO4 ) 2 1molFeNH4 (SO4 ) 2 2molFe
1molFe2 0 3
Percent Error:
0.0135  0.0314
* 100% = 57%
0.0314
Percent Recovery:

Theoretical Yield
 For solving the theoretical
yield, Minitalsolid= 0.1398 g
Theoretical yield = 0.0314 g
Percent Error:
Actual  Expected
*100% = Percent Error
Expected
Pr ecipitateMass
*100%
TheoreticalYield
Trial 3
0.0315
60.3%
39.7%
Percent Recovery:

0.0135
*100% = 43 %
0.0314
* All calculations were based on Trial 1

Discussion:

The most significant source of error in this experiment was human error. The transfer of
product to filtration was a source of error. Not all of the solution was successfully transferred over.
Also, when massing the original unknown, it created a problem because the balance beam fluctuated
the mass and there was no “concluded” mass.
The function of the nitric acid was to “pull out” the iron content in the sample.
The term gravimetric analysis is defined as the set of methods in analytical chemistry, for the
quantitative determination of an analyte based on the mass of the solid.
The modifications that I would improve for this experiment was to have a fitting lid for each
crucible. The reason being is because there were two lids that broke during this experiment. When
the lids broke, it decreased in its original mass and affected the overall mass. Another modification
that I would change would be to minimize the amount of transfer for the sample.
Conclusion:
Overall, the result of this experiment was not successful. The reason being is because the
percent recovery was significantly low. This may have been due to the lids of the crucibles to break
and this may have caused a change in the mass of the precipitate mass.