Determining the empirical Formula of MgO

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Section 1: Purpose and Summary
Determine the empirical formula of magnesium oxide.
Calculate the mass of oxygen using weighing-by-difference.
Calculate the mole of a sample from its mass.

In this experiment, students will conduct the reaction between magnesium and oxygen gas. Students will determine the mass of magnesium sample before and after the reaction, and the mass of magnesium and oxygen in the product. Students will learn how to convert mass to mole of a given sample and determine empirical formula of a substance from mass and mole data.
Section 2: Safety Precautions and Waste Disposal

Safety Precautions:

Do not look directly at the burning magnesium ribbon. The flame is bright enough to damage your eye. Use of eye protection is required for all experimental procedures.
A hot crucible will break if placed directly on a cold surface. Set hot crucibles on to wire screens to cool.
A hot crucible will break if splashed with water directly. Let crucibles cool prior to adding water.
Waste Disposal:
The solid product from the reaction can be disposed into the regular garbage can in the lab.
Section 3: Procedure
Part 1: Preparation of the crucible
Heat a clean and dry porcelain crucible with lid (on a clay triangle supported on a ring stand) directly over a Bunsen burner flame for about 5 minutes. This will ensure that the crucible is clean and dry.
Here is what the setup should look like:
Turn off the burner and allow the crucible to cool. Leave the crucible resting on the clay triangle. DO NOT put a hot crucible on the lab bench.
When the crucible is cool to touch, weigh the crucible and lid on the digital balance.
It is important to use a balance that measures to 0.0001 grams. If you use the wrong balance, you will have to repeat this experiment.
Mass of empty crucible and lid:

(a)________________ grams
Part 2: Preparation of magnesium sample



Obtain a strip or two of magnesium ribbon, about 6.0 to 7.0 cm long.



NOTE: If it is not shiny, polish it slightly with steel wool to remove any oxide coating.



Use sufficient magnesium strips to mass between 0.4 and 0.6 grams on an analytical balance (a balance that measures to 0.0001 g)



Fold or loosely coil the magnesium ribbon(s) to make sure it sits entirely on the bottom of the crucible.
Put the coiled magnesium ribbon into the crucible, cover, and weigh on the digital balance. Record mass.

Mass of crucible, magnesium ribbon, and lid:

(b)________________ grams

Part 3: Heating the magnesium sample
Place the crucible containing the magnesium ribbon back into the clay triangle. Put the lid close by on the lab bench, with crucible tongs to handle it with later.
Light the Bunsen burner and start heating the crucible and magnesium ribbon. As soon as the magnesium ribbon starts to burn, put the lid back (using crucible tongs) to put out the flame.
Continue heating the crucible with the lid on for a minute or so, then remove the lid again. When the magnesium ribbon starts to burn again, put the lid back to put out the flame.
Repeat this heating process until the magnesium ribbon no longer catches fire.
With the lid on, heat the crucible and sample strongly for 5 minutes. Make sure that the bottom of the crucible turns ‘red’ hot.
Turn off the burner, and let the crucible, lid, and sample cool on the clay triangle.
When the crucible is cool to touch, remove the lid, then add about 10 drops of laboratory water into the burned magnesium ribbon. Make sure to wet the entire sample, not just one spot.
Light the Bunsen burner and heat the crucible (without lid) with a low flame for a minute or so. This evaporates the water you just added. After the water has evaporated, increase the flame and heat it strongly for about 10 minutes. The bottom of the crucible does not need to turn ‘red’ hot during this heating.
Turn off the burner, and the let the crucible and the sample cool on the clay triangle.
When the crucible is cool to touch, weigh the crucible, product, and lid on the digital balance.
Mass of crucible, product, and lid:
(c)________________ grams
Section 4: Calculations
From the masses you recorded in Parts 1 and 2 of this experiment, calculate the mass of magnesium ribbon.

(b) – (a)
(d) _________________ g Mg
From the masses you recorded in Parts 2 and 3 of this experiment, calculate the mass of magnesium oxide produced from the reaction.
(c) – (a)

(e) _________________ g MgO
From the mass of magnesium oxide (e) and the mass of magnesium ribbon (d), calculate the mass of oxygen that combined with magnesium during the reaction.
(e) – (d)
(f) ________________ g O

Refer to a periodic table to obtain the molar mass of magnesium. Then, using this information and the mass of magnesium ribbon (d), calculate the number of moles of magnesium that you started with.
Molar mass of Mg: ___________g/mol (from a Periodic Table)