Empirical Formula of Magnesium Oxide
The stoichiometric ratio in which substances combine in a reaction is one of the most important determinations for a chemical process. In this experiment, you will determine the percentage of magnesium in magnesium oxide and the empirical formula of magnesium oxide.
Magnesium metal is a moderately reactive elementary substance. At room temperature, magnesium reacts only very slowly with oxygen and can be kept for long periods of time without appreciable conversion to the oxide. At elevated temperatures, however, magnesium will ignite in an excess of oxygen gas, burning with an intensely white flame and producing magnesium oxide. Because of the brightness of its flame, magnesium is used in flares and in photographic flashbulbs.
In this experiment, however, you will be heating magnesium in a closed container called a crucible, exposing it only gradually to the oxygen of the air. Under these conditions, the magnesium will undergo a more controlled reaction, gradually turning from the shiny free metal to the grayish-white powdered oxide. Because the air also contains a great deal of nitrogen gas, apportion of the magnesium being heating may be converted to magnesium nitride, Mg3N2, rather than to magnesium oxide. Magnesium nitride reacts with water and, on careful heating, is converted into magnesium oxide, however.
Mg3N2 + 3H2O ŕ 3 MgO + 2NH3
The ammonia produced by this reaction can be detected by its odor, which is released on heating the mixture.
Magnesium is a Group IIA metal, and its oxide should have the formula MgO. Based on the formula, magnesium oxide has a specific percentage of magnesium by weight, which you can calculate. By comparing the weight of magnesium reacted, and the weight of magnesium oxide that results from the reaction, this will be confirmed.
You will also calculate an experimental empirical formula.
Porcelain crucible and cover, crucible tongs, clay triangle, magnesium ribbon, pH paper, wash bottle of distilled water, 6 M HCl, steel wool, goggles, apron, fume hood.
Obtain a porcelain crucible and cover, and examine them. The crucible and cover are extremely fragile and expensive. Use caution in handling them.
If there is any loose dirt in the crucible, moisten and rub it gently with a paper towel to remove the dirt. If dirt remains in the crucible, bring it to the hood, add 5-10 mL of 6 M HCl and allow the crucible to stand for 5 minutes.
Discard the HCl and rinse the crucible with water. If the crucible is not clean at this point, consult with the instructor about other cleaning techniques, or replace the crucible.
After the crucible has been cleaned, use tongs or a lint-free tissue to handle the crucible and cover to prevent adsorption of oil from your fingers.
Set up a clay triangle on a ring stand. Transfer the crucible and cover to the triangle. The crucible should sit firmly in the triangle (the triangle’s arms can be bent slightly if necessary).
Begin heating the crucible and cover with a small flame to dry them. When the crucible and cover show no visible droplets of moisture, increase the flame to full intensity, and heat the crucible and cover for 5 minutes.
Remove the flame, and allow the crucible and cover to cool to room temperature.
When the crucible and cover are completely cool, use tongs or a lint-free tissue to move them to a clean dry watch glass or flat glass plate. Weight the crucible and cover to the nearest 0.1 mg using the analytical balance. (0.0001 g). Use this balance for all measurements.
Return the crucible and cover to the clay triangle. Reheat in the full heat of the burner flame for 5 minutes. Allow the crucible/cover to cool completely to room temperature.
Reweigh the crucible after it has cooled. If the weight this time differs from the earlier weight by more than 5 mg (0.0050 g), reheat the crucible for an additional 5 minutes and reweigh when cool. Continue the heating/weighing until the weight of the crucible and cover is constant to within 5 mg.
Place an 8 inch strip of magnesium ribbon on top of a paper towel and polish with steel wool to remove the oxide coating. Coil the strip into a spiral and place in the crucible.
Find mass of crucible/cover and magnesium
Set up crucible on clay triangle with cover very slightly ajar (See Figure 1). With a very small flame, begin heating the crucible gently.
If the crucible begins to smoke when heating, immediately cover the magnesium completely and remove the heat for 2-3 minutes. The smoke consists of the magnesium oxide product and must not be lost from the crucible.
Continue to heat gently for 5-10 minutes with the cover of the crucible slightly ajar. Remove the heat and allow the crucible to cool for 1-2 minutes.
Remove the cover and examine the contents of the crucible. If portions of the magnesium still demonstrate the shiny appearance of the free metal, return the cover and heat with a small flame for an additional 5 minutes; then re-examine the metal. Continue heating with a small flame until no shiny metallic pieces are visible.
When the shiny magnesium metal appears to have been converted fully to the dull gray oxide, return the cover to its slightly ajar position, and heat the crucible with the full heat of the burner flame for 5 minutes.
Slide the cover to about the half-open position and heat the crucible in the full heat of the burner flame for an additional 5 minutes.
Remove the heat and allow the crucible and contents to cool completely to room temperature. Remove the crucible from the clay triangle and set it on a sheet of clean paper on the lab counter.
With a stirring rod, gently break up any large chucks of solid in the crucible. Rinse any material that adheres to the stirring rod into the crucible with a few drops of distilled water.
With a dropper, add about 10 additional drops of distilled water to the crucible, spreading the water evenly throughout the solid.
Return the crucible to the clay triangle, and set the cover in the slightly ajar position.
With a very small flame, begin heating the crucible to drive off the water that has been added. Beware of spattering during the heating. Remove the flame and close the cover of the crucible if spattering occurs.
As the water is driven off, hold a piece of moistened pH paper (with forceps) in the stream of steam being expelled from the crucible. Any nitrogen that has reacted with the magnesium is driven off as ammonia during the heating and should give a basic response with pH paper (you may also note the odor of ammonia).
When it is certain that all the water has been driven off, slide the cover so that it is in approximately the half-open position, and increase the size of the flame. Heat the crucible and contents in the full heat of the burner for 5 minutes.
Allow the crucible and contents to cool completely to room temperature. When completely cool, weigh the crucible and contents.
Return the crucible to the triangle and heat for another 5 minutes in the full heat of the burner flame. Allow the crucible to cool completely to room temperature and reweigh. The two measurements of the crucible and contents should give weights that agree within 5 mg (0.005 g).
If this agreement is not obtained, heat the crucible for additional 5 minute periods until two successive weighings agree within 5 mg.
Calculate the percentage of magnesium in the magnesium oxide from your experimental data. Compare to literature values appropriately. Calculate the experimental empirical formula.