Rust and Other Oxides
Prelab
Name___________________________________________________
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Total________/10
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1. What is the purpose of this experiment?
2. Define an oxide and how it is formed.
3. In this laboratory experiment, two oxides of iron can form. These two oxides are iron(II)
oxide and iron(III) oxide. Write the balanced molecular equations for the reaction of iron
metal reacting with molecular oxygen to produce iron(II) oxide and iron(III) oxide. NOTE:
You should write TWO separate chemical equation.
4. You are going to heat arsenic in the presence of oxygen to form an oxide. An empty crucible
weighs 22.1156 g and after adding some arsenic, the mass of the crucible and arsenic is
25.7791 g. After heating the arsenic and completely converting it to the oxide, the mass of
the crucible and oxide is 27.7340 g. Determine the empirical formula of the arsenic oxide.
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Rust and Other Oxides
In this experiment, you will be able to determine the empirical formula for an iron oxide from
results you will obtain by heating iron metal in the presence of oxygen in the air.
Special mention goes to Brian Stahl, John Witkowski, John Curtin, Nevin Gundez, Kelly Lyons
and Tom Pomorski for developing this laboratory experiment so you can have a better
understanding of the formation of oxides.
Introduction
Whether it is alone or in air, molecular oxygen, O2, is a very reactive substance when it is heated
and many elements will react with it. When an element reacts with molecular oxygen, it
chemically combines with the oxygen and forms an oxide. The element you will be working
with in this laboratory experiment is iron.
Iron comprises approximate 5% of the earths crust and contributes to 35% of the earth’s mass.
Iron forms many alloys with other metals such as nickel, manganese, magnesium and copper. It
also forms alloys with nonmetals such as carbon and silicon in the production of steel. Most of
the iron in the earths crust exists as iron oxides and this is the kind of oxides you will be studying
in this laboratory experiment. Iron can form one of two iron oxides. The first is iron(II) oxide
where the charge on iron is +2 and iron(III) oxide where the charge on the iron is +3. In this
experiment, iron metal in the form of a powder will be heated and react with oxygen to form one
or both of the iron oxides.
You can determine the amount of oxygen that is present in the oxide by taking the mass of the
iron oxide present and the original mass of the iron powder that you weighed when you started
the experiment. For an example calculation, we will have the reaction of phosphorous with
molecular oxygen to form some phosphorous oxide. In this example, we will have 0.1920g of
phosphorous burn in the presence of oxygen and produce 0.4400g of the oxide. The first step is
to determine the mass of the oxygen in the oxide. We obtain this value by subtracting the mass
of the phosphorous in the beginning from the mass of the oxide.
0.4400g – 0.1920g = 0.2480g of oxygen
The next step is to convert both the mass of the phosphorous and oxygen to moles.
0.1920g P x 1mole P = 6.199 x 10-3 moles of P
30.974g P
0.2480g O x 1 mole O = 1.550 x 10-2 moles of O
16.00g O
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Next, you want to divide each value you have obtained by the smallest number of moles.
6.199 x 10-3 = 1.000
6.199 x 10-3
1.550 x 10-2 = 2.500
6.199 x 10-3
Since you want your empirical formula to have whole positive numbers, you must multiply these
numbers by two. This gives you the empirical formula for the phosphorous oxide produced as
P2O5.
Procedure
1. Obtain a stainless steel crucible (no lid). Brush the crucible with a paper towel to get rid of
any soot. DO NOT WET THE CRUCIBLE!!!!!!!!!!
2. Place the EMPTY crucible in a clay triangle on an iron ring which is attached to a ring stand.
Adjust the height of the iron ring so that the bottom of the crucible will be in the hottest part
of the flame from your bunsen burner. If you are unsure of your setup, have your instructor
check it for you.
3. Heat the crucible strongly for about five minutes. Make sure that the hottest part of the flame
just touches the bottom of the crucible. The bottom of your crucible should have a red-hot
glow during this time. After five minutes have passed, move the burner from under the
crucible and allow the crucible to cool for three to five minutes. You should not feel any heat
when you place the back of your hand or the inside of your wrist about a half inch from the
bottom of the crucible when it is cooled. Do not touch the crucible when you do this.
4. Take the cooled crucible and transfer it to the pan of a balance using crucible tongs and wire
gauze to place under the crucible. If you have to wait in line for a balance, DO NOT put your
crucible on the lab bench. Rest the crucible on your wire gauze.
5. Obtain and record the mass of your crucible. Remember to use the same balance each time
you obtain a mass for your crucible.
6. Repeat steps 3, 4 and 5 until two CONSECUTIVE masses differ by no more than  0.0030g.
When you have achieved this, record this final mass and move on to step seven.
7. Weigh out approximately 0.0300-0.0600 g of iron powder and place in the crucible. Weigh
the crucible with the iron powder in it. The difference between the empty crucible and the
mass of the crucible and iron will determine the amount of iron in the crucible. DO NOT
WRITE THE MASS OF THE IRON THAT YOU WEIGHED BEFORE PLACING IT IN
THE CRUCIBLE.
8. Set up your crucible the same way as in step 2. Heat the crucible strongly again for
approximately 15 minutes.
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9. After 15 minutes has passed, allow the crucible to cool as before in step 3.
10. When the crucible is cooled, obtain the mass of the crucible and record it.
11. Heat the crucible strongly again for around three minutes. Cool as before and obtain the
mass of the crucible and record it.
12. If your two masses do not differ by more than  0.0030g then you are finished and you can
dispose of the contents properly and clean the crucible (just wipe with a dry paper towel). If
the difference is greater than 0.0030g then repeat step 11 until two CONSECUTIVE masses
meet the requirements.
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Results
Mass of empty crucible
1st Heating
2nd Heating
3rd Heating 4th Heating
(if necessary) (if necessary)
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Last reading of mass of empty crucible
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Mass of crucible and Fe
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Mass of Fe
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Mass of crucible and oxide
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Last reading of mass of crucible and oxide
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Mass of Oxide
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Mass of Oxygen in Oxide
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Moles of Fe in Oxide
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Moles of Oxygen in Oxide
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Empirical Formula of Oxide
(according to your data)
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Calculations
Questions
1. Write the balanced molecular equations for the reaction of iron with molecular oxygen to
form both iron(II) and iron(III) oxide. (You should have two equations written).
2. Calculate the mass (in grams) of iron(II) oxide and iron(III) oxide that can be produced from
the amount of iron you weighed out in step 7. (You should have two values, one for iron(II)
oxide and one for iron(III) oxide).
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