Experiment #2 – Gravimetric Determination of Iron as Fe2O3
Introduction:
By precipitating hydrated iron oxide from a basic solution, students will analyze
the amount of iron in the given sample. The equations carried out in the experiment are:
Fe3+ + 3H2O FeOOH*H2O(s) + 3H+
2FeOOH*H2O  Fe2O3(s) + 2H2O
Procedure:
Three porcelain crucibles with lids were heated and weighed. Three samples of an
unknown iron-containing compound were weighed to produce about 0.03 g of iron. The
unknown samples were dissolved in hydrochloric acid, filtered and boiled. The
supernatant from each unknown sample was decanted with ashless filter paper into the
three crucibles. The filter paper was dried for about 48 hours and then the filter paper was
ignited. The three solid samples were then cooled and weighed.
Data:
Porcelain crucible with Lid
Blue
Red
Green
(g)
43.7722
39.2602
42.0021
Porcelain crucible with Lid (after drying)
Blue
Red
Green
(g)
42.0099
41.2902
42.0108
Wire Mesh (with Porcelain crucible with Lid)
Blue
Red
Green
Samples of Unknown #158
#1 (clear)
#2 (yellow)
#3 (brown)
Sample
#1
#2
#3
(g)
62.8517
59.7376
61.4669
Filter paper
#1
#2
#3
(g)
0.1857
0.2011
0.1783
Ammonium added (mL)
~11
~11
~11
color change
yellow
orange
darker orange
(g)
0.2313
0.2348
0.2294
Final Weight (weighed 4 times) (g)
Blue
Red
Green
#1
42.1341
39.3058
43.8056
#2
42.1351
39.3067
43.8064
#3
42.1355
39.3070
42.8070
#4
42.1356
39.3073
43.8072
average
42.1351
39.3067
43.8066
Calculations:
1 𝑚𝑜𝑙 𝐹𝑒2𝑂3
2 𝑚𝑜𝑙 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
256.975 𝑔 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
0.03𝑔 × 159.691 𝑔 𝐹𝑒2𝑂3 × 1 𝑚𝑜𝑙 𝐹𝑒2𝑂3 × 1 𝑚𝑜𝑙 𝑜𝑓 𝑢𝑛𝑘𝑛𝑜𝑤𝑛 = 0.1 𝑔 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
With the assumption that the unknown contains ~75% ferrous ammonium sulfate:
0.1𝑔 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
= 0.133 𝑔 𝑢𝑛𝑘𝑛𝑜𝑤𝑛
0.75 𝑓𝑒𝑟𝑟𝑜𝑢𝑠 𝑎𝑚𝑚𝑜𝑛𝑖𝑢𝑚 𝑠𝑢𝑙𝑓𝑎𝑡𝑒
Post Lab Questions:
A. The biggest source of error was losing product due to changing glassware. For
example, we moved the solution from Erlenmeyer flasks to beakers. Our results
justify this because we had less than we expected.
B. Nitric acid is used to precipitate the iron out of the solution in step 3.
C. Gravimetric analysis is the method of determining an analyte based on the mass
of a solid using quantitative properties.
D. Specify the container in which to put the solution and specify how to burning the
paper in the crucible, oven vs. Bunsen burner.
Conclusions:
The experiment was a good representation of the technique of gravimetric
analysis. Although some of the results varied greatly (sample 2), the majority of them
were sound. It proves that this is a decent demonstration of analysis. Sources of error
were all human. There would be no changes to this lab in the future to ensure better
results.