Experiment 2
Gravimetric Determination of Iron as Fe2O3
Purpose: In this lab, we will be precipitating hydrated iron oxide from a basic solution and then
dehydrating the result in order to analyse the amount of iron in the compound.
Procedure:
1. Heat three crucibles and caps to redness for 10 minutes over a Bunsen burner in the hood. Cool
for 15 minutes and weigh.
2. Weigh three samples of unknown to produce around 0.03 g of iron product. Dissolve each
sample in 3 M HCl using heat if necessary. If there are impurities, filter. Add 5 mL 6 M HNO3
and boil for 2 min.
3. Dilute sample to 200 mL with distilled H2O and add 3 M ammonia until basic. Digest precipitate
with boiling for five minutes.
4. Decant supernatant through ashless filter paper. Wash the solid with 1% NH4NO3 until no Cl- is
detected. Then acidify with dilute nitric and silver nitrae.
5. Allow filter aper and solid to dry for at least 24 hours. Protect from dust.
6. Dry the sample and ignite filter paper using clean crucibles in the hood. The paper should be
ignited slowly and the flames quenched immediately by covering with lid. Upon cooling, weigh
the solid.
Data**
Empty Crucibles +
Lids
Crucible 1 (g)
Crucible 2 (g)
Crucible 3 (g)
Trial 1
Trial 2
Trial 3
Average
52.7133
49.7413
56.6455
52.7113
49.7396
56.6452
52.7112
49.7399
N/A
52.7119
49.7403
56.6454
Unknown Sample
Sample 1 (g)
Sample 2 (g)
Sample 3 (g)
Weigh 1
0.2050
0.1967
0.1974
Weigh 2
0.2047
0.1967
0.1963
Weigh 3
0.2043
0.1966
0.1962
Average
0.2047
0.1967
0.1966
Sample + Crucible +
Lids
Weigh 1
Weigh 2
Average
Sample/Crucible 1 (g)
52.7308
52.7303
52.7306
Sample/Crucible 2 (g)
49.7604
49.7608
49.7606
Sample/Crucible 3 (g)
56.6581
56.6577
56.6579
**Data taken from Sam Yatzkan and Ronnie Groller because of experimental difficulties on our part.
Iron Product
Sample 1
Sample 2
Sample 3
Weight (g)
0.0187
0.0203
0.0125
Percent Yield
62.3%
67.7%
41.7%
Calculations
Approximate amount of unknown needed to make 0.03g of Fe2O3:
0.03𝑔 𝐹𝑒2 𝑂3 ∗
1 𝑚𝑜𝑙 𝐹𝑒2 𝑂3
2 𝑚𝑜𝑙 𝐹𝑒𝑂𝑂𝐻 1 𝑚𝑜𝑙 (𝑁𝐻4 )2 𝐹𝑒(𝑆𝑂4 )2 ∗ 6𝐻2 𝑂 1
∗
∗
∗
=
159.688𝑔𝐹𝑒2 𝑂3 1 𝑚𝑜𝑙𝐹𝑒2 𝑂3
1 𝑚𝑜𝑙 𝐹𝑒𝑂𝑂𝐻
. 75
0.196𝑔 𝑜𝑓 𝑢𝑛𝑘𝑛𝑜𝑤𝑛 𝑛𝑒𝑒𝑑𝑒𝑑 𝑡𝑜 𝑝𝑟𝑜𝑑𝑢𝑐𝑒 0.03𝑔 𝑜𝑓 𝐹𝑒2 𝑂3
Average:
𝑋1 + 𝑋2 + 𝑋3 =
Ex) 0.2050 + 0.2047 + 0.2043 =
0.614
3
𝑋𝑡𝑜𝑡𝑎𝑙
𝑛
= 0.204667 (𝑅𝑜𝑢𝑛𝑑𝑒𝑑 𝑡𝑜)0.2047
Weight of final product: Weight of Crucible, Lid, and Product – Weight of empty Crucible and Lid
𝐴𝑐𝑡𝑢𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡
Percent Yield: 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 × 100% Ex) 𝑠𝑎𝑚𝑝𝑙𝑒 1:
0.0187𝑔
0.03𝑔
× 100% = 62.3%
Conclusions:
The percent yield for the other group’s sample was obtained was quite significant, being that two of the
trials yielded over 50% (62.3% and 67.7%, respectively). The last trial gave a 41.7% yield, most likely due
to some sort of slip up or difficulty.
Our results did not turn out as we added the acid to the solution in the wrong place, adding to the filter
paper instead of the solution in the filter flask. Due to this mistake, we were required to use another
group’s data instead of our own.
Post-Lab Questions:
1. What do you consider the single most significant source of error in this experiment? Do your
results justify this answer?
Even though we did not technically complete the lab, I would hazard a guess
that weighing the crucibles would turn out to have the most error. They were
all weighed and heated at different points for different lengths of time, making
the data inconsistent.
2. What was the function of the nitric acid in step two?
The nitric acid, along with boiling the solution, helps to precipitate the iron out
of the initial solution.
3. What does the term gravimetric analysis mean?
Gravimetric analysis is the quantitative analysis of a substance based on the
solid. In other words, it is the analysis based on the final weight of a sample.
4. What modifications would you make to improve this experiment?
If I could change anything in this experiment, I would make the directions a bit more
clear. It was confusing to us where to add certain chemicals such as the acid at the very
end of the experiment. Also, I would suggest heating the crucibles to a specific
temperature instead of heating them until “redness” to induce more accurate results as
the crucible weight seemed to account for some of the error.
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