NAME _______________________
CHEM-1111 General Chemistry I
Experiment 3A: The Empirical Formula of an Oxide
Introduction
The formula of a chemical compound can be determined by a combination of experimental analysis and
calculations.
The empirical formula represents the simplest formula for a compound. The empirical formula is
typically used for ionic compounds.
When an element reacts with oxygen, an oxide is formed. Molecular oxygen is very reactive and can react
with a great deal of elements and compounds. Even though nitrogen is present in much larges amounts in
the atmosphere, the reactivity of nitrogen is much lower than the reactivity of oxygen
Concept of the experiment
When magnesium is burned in air, it will form magnesium oxide (MgO) and a small amount of
magnesium nitride (Mg3N2).
Magnesium nitride is converted to magnesium hydroxide (Mg(OH)2) and ammonia (NH3) in water.
Heating will result in the conversion of Mg(OH)2 to MgO with the loss of water vapor.
The reaction product will now consist entirely of MgO. The amount of oxygen in the oxide can be
determined from the mass of the oxide and the original amount of magnesium that you started with.
The Law of Mass Conservation states that the amount of reactants equals the amount of product,
therefore the mass of magnesium oxide (the “product”) is equal to the amounts of magnesium and oxygen
(“reactants”).
Using molar conversion based on the molar mass of the magnesium used and the amount of magnesium
oxide formed, the molar amount of oxygen used can be determined and therefore the empirical formula of
magnesium oxide.
1
NAME _______________________
Procedure
Getting Started
1) Get one crucible and lid per student couple. Wash, rinse and dry them. Set aside
2) Obtain about 0.2 g magnesium ribbon (weigh on balance)
3) Place crucible on clay triangle on iron stand
Experimental
1) Heat the crucible for about 3 minutes until it glows red. Allow the crucible and lid to cool for
about 5-10 minutes
2) When the crucible has reached room temperature, determine the mass of crucible and lid on a
balance.
3) Repeat the heating, cooling and weighing until the recorded masses are within 0.001 g of each
other. Calculate and record the average of the masses.
4) Fold the magnesium ribbon in a loose ball and place it inside the crucible
5) Cover the crucible with the lid and determine the mass of the crucible, lid and magnesium ribbon
6) Put the crucible on the clay triangle and gently heat it for about 2-3 minutes. Next heat the
crucible in the hottest part of the flame for about 3 minutes
7) Using tongs, lift the lid slightly to allow more air in. DO NOT LIFT THE LID COMPLETELY to
avoid burning the magnesium
8) Repeat this step until all the magnesium has disappeared and no glow is present when lifting the
lid.
9) Allow the crucible to cool
10) Add a few drops of dH2O to the content. Do you smell anything?
11) Cover the crucible with the lid slightly ajar. Heat until the contents is dry (WHY?).
12) Heat the crucible for about 8-10 minutes to convert hydroxide to oxide
13) Let the crucible cool down to room temperature
14) Obtain and record the mass of the crucible
15) Heat for another 3 minutes and let the crucible cool
16) Repeat the heating, cooling and weighing until the recorded masses are within 0.001 g of each
other. Calculate and record the average of the masses.
17) Clean the crucible and lid. Do not break!!
2
NAME _______________________
Calculations
1) Mass of empty crucible + lid
__________ g
__________ g
__________ g
__________ g
2) Average mass of empty crucible + lid
___________ g
3) Mass of crucible, lid and Mg
___________ g
4) Average mass of empty crucible + lid
___________ g (from 2)
5) Mass of magnesium
___________ g
6) Mass of crucible, lid and magnesium oxide __________ g
__________ g
__________ g
__________ g
7) Average mass
___________ g
8) Average mass of empty crucible + lid
___________ g (from 2)
9) Mass of magnesium oxide
___________ g (7 – 8)
10) Mass of magnesium
___________ g (from 5)
11) Mass of oxygen used in reaction
___________ g (9 - 10)
12) Molar mass of magnesium = mass of Mg (g)/molar mass of Mg (g/mol)
_______ mol
13) Molar mass of oxygen = mass of O (g)/molar mass of O (g/mol)
_______ mol
14) Ratio of magnesium and oxygen = x mol Mg / y mol O= x/y
15) Empirical formula = MgxOy
Questions
Answer questions 1, 2a-d
3