F.6/7 Chemistry Manual: Empirical Formula of Magnesium Oxide
Objective: To determine the empirical formula of magnesium oxide by weighing method
Size of group: Individual
Introduction:
In this experiment, a certain quantity of magnesium is weighed. The magnesium is burned in a
crucible. Oxygen in the air combines with the magnesium to form magnesium. Mass of the
product, MgO, is measured and mass of oxygen in the compound is calculated. Hence number
of moles of magnesium and oxygen are determined and the mole ratio of the element can be
determined. By using this ratio the empirical formula of magnesium is estimated.
-----------------------------------------------------------------------------------------------------------------Theory: Give principle/reason to explain how is the formula of magnesium oxide
determined in this experiment. The following questions should be useful to you.
1. Compare the reactivity of magnesium with other metals in the metal reactivity series.
2. What is the method employed in this experiment to find the masses of magnesium and
oxygen for the magnesium oxide in a crucible?
3. Suppose the mass of magnesium strip is m1, mass of the product is m2, show, how do you
find out the mole ratio of magnesium and oxygen in magnesium oxide.
How can you find out the empirical formula from the mole ratio?
4.
----------------------------------------------------------------------------------------------------------------Chemicals:
Magnesium strip
Additional Materials:
Sand paper, balance readable 2 decimal places, crucible tong
Procedure
Record your results immediately after you get the readings.
Wear goggle in the course of the practical
1.
Weigh a dry clean crucible and lid with a small pad of
ceramic wool at the bottom. The pad is to protect the
surface of crucible from the action of burning
magnesium.
2.
3.
4.
5.
6.
7.
You will be given a magnesium ribbon of about 10 cm long. Remove any surface
coating of oxide with sand paper. Coil the ribbon around a pencil and place the
ribbon on the pad inside the crucible. Weigh the crucible, lid and contents.
Support the crucible on a pipe-clay triangle placed on a tripod. Heat the crucible for
about 2 to 3 minutes using a non-luminous Bunsen flame. Make sure the flame is low
at first and then gradually increase it. Use crucible tongs to lift the lid occasionally.
Continue heating and raise the lid at intervals until the magnesium catches fire. When
this happens, remove the burner and keep the magnesium burning by lifting the lid
every minute. In doing so, the lid should be lifted slightly but kept level just over the
rim of the crucible.
When the magnesium no longer burns, remove the lid and place the crucible on a
wire gauze with ceramic square. Heat it strongly for 5 minutes.
Replace the lid and allow the crucible to cool for at least 20 minutes.
Reweigh the crucible, lid and contents.
Data and Results
Mass of crucible, lid and pad
= ____________ g
Mass of crucible, lid and pad + magnesium = ____________ g
Mass of crucible, lid and pad +
magnesium oxide
= ____________ g
Mass of magnesium used
= ____________ g
Mass of oxygen which combines with this
mass of magnesium
Mole2: Formula of magnesium oxide/p.1
= ____________ g
Calculation
1.
2.
Calculate number of moles of magnesium and oxygen in the magnesium oxide.
Calculate relative number of moles of magnesium and oxygen in the magnesium oxide. Hence find out
the empirical formula of magnesium oxide.
Questions for discussion
1. List safety precautions for this practical.
2. Knowing that the empirical formula of magnesium oxide is MgO, calculate percentage error for your
results. Do you accept your results?
3. List all sources of experimental error.
4. If there is magnesium unreacted in the crucible, suggest a method to confirm the presence of this
unreacted magnesium.
5. In step 3, the crucible lid is lifted occasionally. In step 4, the lid is lifted slightly. Why are they
necessary?
6. What are the safety precautions in this experiment?
7. Can this method apply to the determination of formula of (a) copper(II) oxide and (b) calcium oxide?
Explain.
Mole2: Formula of magnesium oxide/p.2