THE EMPIRICAL FORMULA OF MAGNESIUM OXIDE

 

Purpose

 

The purpose of this investigation is to determine the empirical formula of magnesium oxide by burning a known mass of magnesium in air, and using the known atomic masses of magnesium and oxygen to find the simplest whole number molar ratio of the elemental constituents.

 

 

Instructions

 

Obtain a twenty-five centimeter sample of magnesium metal (about 0.2 grams) and a cruci­ble with lid.  Clean the crucible and lid by heating it on a clay triangle over a Bunsen burner until the bottom of the crucible glows orange, then allow the crucible to cool.  Instructions will be given on how to correctly handle the crucible with crucible tongs.  Be sure not to touch the crucible or lid with your fingers during the activity, which will affect the mass of the crucible and its contents.

 

Find the mass of the magnesium to the closest 0.001 gram.  Weigh the cooled crucible as well.  Cut up the magnesium strip into small pieces and add them to the crucible.  Calculate the moles of magnesium by dividing the mass in grams by the atomic weight of magne­sium, 24.30 grams/mole, and report the moles (or millimole­s, mmol) of magnesium.

 

Place the crucible on a clay triangle and heat it and its contents with a Bunsen burner.  The lid should be on the crucible to prevent the loss of magnesium oxide ash, but it should also be set to allow air to enter the crucible.  Lift the lid slightly and briefly every five minutes to allow more air in.  If the magnesium metal glows brighter when the lid is lifted then the reaction is not complete.  The reaction should take around 20-30 minutes.

 

When all of the magnesium has reacted, cool the crucible and lid and weigh them again.  The added mass compared to the empty crucible is the mass of the product.  The difference between the mass of product and the original mass of magnesium is the mass of oxygen added by the reaction.  Calculate the moles of oxygen by dividing the mass by 16.00 grams/mole, and report the moles (or millimole­s, mmol) of oxygen.

 

Find the ratio of magnesium to oxygen in the sample.  Write the actual number of millimoles of magnesium to millimoles of oxygen as  mmol Mg : mmol O  then divide both numbers by the smallest value.  The ratio will now be expressed as either  1.00 : x.xx  or  x.xx : 1.00.  Use this data to report an empirical formula for magnesium oxide.

 

 

Preliminary Question

 

A crucible weighs 35.346 grams empty, 35.561 grams containing magnesium metal, and 35.841 grams after a reaction with sulfur to produce magnesium sulfide.  What is the empirical formula of magnesium sulfide?

 

 

 

 

 

 

 

 

Results

 

 

1.     Mass of magnesium:

 

 

2.     Moles of magnesium (mass of magnesium/24.3):

 

 

3.     Mass of crucible and lid:

 

 

4.     Mass of crucible, lid, and product (after heating):

 

 

5.     Mass of product (Step 4 – step 3):

 

 

6.     Mass of oxygen (mass of product – mass of magnesium):

 

 

7.     Moles of oxygen (mass of oxygen/16.0):

 

 

8.     Ratio of moles of magnesium to Oxygen:

 

 

(Divide both numbers by the smallest number to get empirical formula).

 

9. Formula of Magnesium hydroxide: