CHEMISTRY LAB
DETERMINATION OF AN
EMPIRICAL FORMULA for Magnesium Oxide
INTRODUCTION:
In this experiment, you will determine the empirical formula of a compound. In so doing you
will gain a clear understanding of the difference between an empirical formula and a molecular
formula.
An empirical formula gives the simplest whole number ratio of the different atoms in a
compound. The empirical formula does not necessarily indicate the exact number of atoms in a
single molecule. This information is given by the molecular formula. For certain compounds,
the empirical formula and the molecular formula are the same; for other compounds the
empirical and molecular formulas are different. In all cases, the molecular formula is a simple
multiple of the empirical formula.
Experiments have shown that any sample of pure water contains two atoms of hydrogen for every
atom of oxygen. The empirical formula for water is, therefore, H2O. Various molecular
formulas; H2O, H4O2, H6O3, and so on, are possible. Each of these formulas expresses the same
ratio of hydrogen and oxygen atoms as is expressed in the empirical formula. Scientists have
shown, however, that each water molecule actually consists of two atoms of hydrogen bound to
one atom of oxygen. Therefore, the molecular formula for this compound is H2O. For water, the
empirical and molecular formulas are identical.
For other compounds, the empirical and molecular formulas are different. Consider hydrogen
peroxide. This compound contains one atom of hydrogen for each atom of oxygen, and its
empirical formula is HO. There is, however, no stable molecule having this formula. In fact, it
has been shown that individual hydrogen peroxide molecules contain two atoms of hydrogen and
two atoms of oxygen. The molecular formula of this compound is, therefore, H2O2.
In some cases, two or more different compounds share the same empirical formula. This is true
of acetylene and benzene. Each of these compounds has the empirical formula CH. The
molecular formula of acetylene, however, is C2H2, while that of benzene is C6H6.
OBJECTIVE:
In this experiment, you will determine the empirical formula of magnesium oxide, a compound
that is formed when magnesium metal reacts with oxygen gas. In determining this empirical
formula, you will make use of the law of conservation of mass.
According to this law, the total mass of the products of a chemical reaction must equal the total
mass of the reactants.
mass of Mg + mass of O2 = mass of MgxOy
Therefore, knowing the mass of magnesium used and the mass of magnesium oxide produced in
this reaction, you can determine the mass of oxygen used. This ratio between the number of
moles of magnesium used and the number of moles of oxygen consumed can then be calculated
and the empirical formula of magnesium oxide can be written on the basis of this ratio.
MATERIALS:
Safety goggles
Bunsen burner
Balance (to 0.01g)
Crucible
Magnesium ribbon, Mg
Crucible lid
Crucible tongs
Ring stand
Ring support
Clay triangle
SETUP:
SAFETY:
 Safety goggles must be worn at all times! Handle the crucible only with the tongs. There
is a significant burn hazard associated with the handling of crucibles because a hot
crucible looks exactly like a cold crucible.
 Remove the gas burner from beneath the crucible before using the crucible tongs to
remove the crucible and its lid. Use the tongs to grasp the lid by its porcelain knob; the
crucible should be grasped by its edge.
 Magnesium, when lit, gives off a bright white light. DO NOT look, peer, stare directly at
the light given off. It can cause blindness and or detach retinas (seriously, it can, it’s that
bright).
PROCEDURE:
1. Place the crucible on a clay triangle balanced on a ring support clamped to a ring stand. Light
the gas burner and adjust it to give a clear blue flame (A yellow flame will deposit soot on the
crucible and cause a large error in your data.). Place the burner under the crucible. Adjust the
height of the ring support so that the bottom of the crucible is in the hottest part of the flame.
Place the crucible lid slightly ajar on the crucible (The crucible lid should be large enough to fit
loosely down over the crucible edge.). NOTHING SHOULD BE IN YOUR CRUCIBLE AT
THIS POINT!
2. Heat the crucible so that its bottom glows red for five minutes. Remove the burner and allow
the crucible and crucible lid to cool. This will take at least 10 minutes. CAUTION: The crucible
gets extremely hot. Never touch it. Always use crucible tongs in handling this piece of equipment.
When the crucible and lid are completely cool, use crucible tongs to transfer them to a balance.
DO NOT place a hot crucible on the balance! Inaccurate mass readings and damage to the
balance may result. Determine the mass of the empty crucible and lid to the nearest 0.01 g.
Record this mass in the DATA TABLE.
3. Coil a 20-cm length of magnesium ribbon and place it in the bottom of the crucible. Determine
the combined mass of the crucible, lid, and the magnesium. Record this mass in the DATA
TABLE.
CAUTION: Do not look directly at the burning magnesium. The intense light may hurt your eyes!
4. Place the crucible, without its lid, on the clay triangle. Heat the crucible strongly until the
magnesium ignites.
CAUTION: Be careful to keep the crucible at arm's length at all times. Do not inhale the
"smoke" produced.
When the magnesium begins to burn, immediately place the cover on the crucible (using tongs)
and remove the burner for a moment.
5. After the reaction has subsided and "smoke" production has ceased, replace the burner and
continue to heat the crucible. Every 2 or 3 minutes, check the progress of the reaction by using
tongs
to
lift
the
lid
of
the
crucible.
Then replace the lid and again apply heat. After 5-8 minutes of heating, remove the burner and
check the product. When the reaction is completed, the magnesium should be completely
converted to a light gray powder, known as magnesium oxide.
If no ribbon-like, shiny gray material remains in the crucible, place the crucible on your tile to
cool completely. If ribbon-like material remains, heat the covered crucible an additional 3-5
minutes, then let the crucible cool completely.
6. After your crucible and its contents have cooled completely find the combined mass of the
crucible, crucible lid, and magnesium oxide. Record in the DATA TABLE.
DATA TABLE:
Mass of crucible and lid = _______ g
Mass of the crucible, crucible lid, and the magnesium (before heating) = _______ g
Mass of the crucible, crucible lid, and magnesium oxide (after heating) = _______ g
CALCULATIONS:
 Determine the mass of magnesium ribbon used in the experiment by subtracting the
mass of the crucible and lid from the mass of the crucible, lid, and magnesium.
Mass of magnesium. = _______ g
 Determine the number of moles of magnesium used. Remember: mass / molar mass = number
of moles. The molar mass of magnesium is 24.3 g / mole .
Number of moles of magnesium = _______ mol
 Determine the mass of magnesium oxide that was formed by subtracting the mass of the
mass of the crucible and lid from the mass of the crucible, lid, and magnesium oxide.
Mass of magnesium oxide formed = _______ g
 Determine the mass of oxygen that combined with the magnesium. Mass of oxygen =
mass of magnesium oxide - mass of magnesium
Mass of oxygen that combined with the magnesium = _______ g
 Determine the number of moles of oxygen that were added to the magnesium to form the
magnesium oxide. This is elemental oxygen so use 16.0 g / mole for the molar mass.
Number of moles of oxygen = _______ mol
 Calculate the ratio between moles of magnesium atoms used and moles of oxygen
atoms used. Remember, this is a simple division. Divide the number of moles of
magnesium by the number of moles of oxygen. Round your answer to the nearest whole
number, as we do not use part of an atom. This represents the moles (and also atoms) of
magnesium. The moles (and also atoms) of oxygen, are represented by 1, because it was
on the bottom of the division.
Moles of Magnesium : Moles of Oxygen
_______
:
___1___
 Give the empirical formula for magnesium oxide that is indicated by your experimental
data.
Empirical formula of magnesium oxide = __________
QUESTIONS:
1. What are the major sources of error in this experiment? (HINT: A major byproduct of this
reaction is between magnesium and the major component of our atmosphere!)
2. How is the law of conservation of mass used in this experiment?
MAIN QUESTIONS:
In your conclusion, you must address the following:
 How did you determine the empirical formula for the magnesium oxide in this experiment?
 Finally, compare your experimental empirical formula for magnesium oxide with the actual
empirical formula for magnesium oxide given to you by your teacher.