Determining the Empirical Formula of an Oxide
Prelab
Name____________________________________
Total_______/10
1. What is the purpose of this experiment?
2. Write the balanced molecular equation for the reaction of calcium with molecular oxygen
to form calcium oxide.
3. You start out with an empty crucible weighing 19.3350g. After adding some calcium to the
crucible, the mass is now 22.1156g. After heating the calcium and completely converting it to
the oxide, the mass of the crucible is now 25.7701g. Determine the mass of the calcium and
oxygen in the oxide.
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Determining the Empirical Formula of an Oxide
In this experiment, you will be able to determine the empirical formula for magnesium oxide
from results you will obtain by burning magnesium in air.
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.
Since you are going to burn the magnesium in the presence of air, you will not only form
magnesium oxide, but you will also be forming magnesium nitride. Since magnesium is an
active metal, it will react with both molecular oxygen and nitrogen in the air. The amount of
magnesium oxide produced will be much greater than the amount of magnesium nitride.
However, you will still have to convert that small amount of magnesium nitride to magnesium
oxide. You will do this by adding water and then heating. The water will convert the
magnesium nitride to magnesium oxide and the nitrogen will be liberated as ammonia. Heating
will convert magnesium hydroxide to magnesium oxide by losing water vapour. Now your
product will consist solely of magnesium oxide.
You can determine the amount of oxygen that is present in the oxide by taking the mass of
magnesium oxide present and the original mass of the magnesium 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
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
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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 crucible and lid. Brush them with a paper towel to get rid of any soot. DO NOT
WET THE CRUCIBLE OR LID!!!!!!!!!!
2. Place the covered 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 covered crucible for about three minutes. Start heating the bottom slowly at first,
brushing the bottom of the crucible with the flame of the Bunsen burner. After you have done
this for about thirty seconds, heat the crucible strongly. The bottom of your crucible should
have a red-hot glow during this time. After this time, move the burner from under the
crucible and all the crucible to cool for five to ten minutes, keep the crucible covered. You
should not feel any heat when you place one of your fingers about a half inch from the bottom
of the crucible when it is cooled. Do not touch the crucible with your finger when you do
this.
4. Keeping the crucible covered at all times, take the cooled crucible and transfer it to the pan of
a balance using crucible tongs and wire gauze or watch glass to place under the crucible. If
you have to wait in line for a balance, DO NOT place your crucible on the lab bench. Rest the
crucible on your wire gauze or watch glass.
5. Obtain and record the mass of your covered crucible. Remember to use the same balance
each time you obtain a mass for your covered crucible.
6. Repeat steps 3, 4 and 5 until two CONSECUTIVE masses differ by no more than ± 0.0010g.
When you have achieved this, record this final mass and move on to step eight.
7. Obtain a piece of magnesium ribbon. If it is not bright and shiny, clean the surface with some
sandpaper.
8. Fold the magnesium ribbon into a loose ball that is able to fit completely in the crucible.
When you are taking the cover off the crucible, be sure to use a paper towel or crucible tongs
since oils from your hand will alter the mass of your crucible. Be careful not to fold the
ribbon too tightly. The best results will be obtained when the surface of the magnesium
ribbon is as exposed as possible.
9. Place the ribbon in the crucible, cover it and obtain and record the mass.
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10. Set up your crucible the same way as in step 2, make sure the lid is completely in place.
Brush the bottom of the crucible with the flame for about two to three minutes. After this
time, place the bunsen burner under the crucible so the bottom of the crucible is in the
hottest part of the flame and heat for another three minutes.
11. Use your crucible tongs to carefully lift the lid by a very small amount to allow some air
to enter the crucible. Do not open the lid too far, if this happens, the metal will enflame.
You want the metal to glow brightly without flames. If you have flames, they will carry part
of the solid magnesium oxide out of the crucible and into the air as smoke. This will result in
very poor data and the wrong empirical formula for your oxide.
12. Repeat step 11 every few minutes until all of the metal is converted to oxide and nitride.
There should be no glow when the crucible lid is lifted.
13. After your conversion is complete, allow the covered crucible to cool as before in step 3.
14. Remove the lid and place it on your wire gauze or watch glass. The contents of the crucible
should look white or slightly grey. Add a few drops of water from a medicine dropper
directly on the contents being careful not to send the contents out of the crucible. You may
smell ammonia at this point.
15. Leaving the lid off the crucible, brush the crucible with the flame to heat it and do this until
the contents inside the crucible are dry. After the contents are dry, close the lid and heat the
crucible strongly for eight to ten minutes. This will convert all of the hydroxide to oxide.
16. Cover the crucible and allow the contents to cool to the same point you did in step 3.
17. When the crucible is cooled, obtain the mass of the covered crucible and record it.
18. Heat the covered crucible strongly again for around three minutes. Cool as before and
obtain the mass of the covered crucible and record it.
19. If your two masses do not differ by more than ± 0.0010g then you are finished and you can
dispose of the contents properly and clean the crucible and lid. If the difference is greater
than 0.0010g then repeat step 18 until two CONSECUTIVE masses meet the requirements.
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Results
Mass of empty crucible and lid
1st Heating
2nd Heating
3rd Heating 4th Heating
(if necessary) (if necessary)
_________
_________
_________
Last reading of mass of empty crucible and lid
_________
Mass of crucible, lid and Mg
_________
Mass of Mg
_________
Mass of crucible, lid and oxide
_________
_________
_________
Last reading of mass of crucible, lid and oxide
_________
Mass of Oxide
_________
Empirical Formula of Oxide
(according to your data)
_________
Calculations
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_________
_________
Questions
1. Write the balanced molecular equation for the reaction of magnesium with molecular oxygen
to form magnesium oxide.
2. Explain the possible reasons for not having data that would agree with the correct empirical
formula. (Answer this question even if you have the correct empirical formula.)
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