Empirical Formula of a Compound
Introduction
The empirical or simplest formula of
a compound gives the smallest whole number ratio of atoms in that compound. In
this experiment you will determine the empirical formula of magnesium oxide. To
illustrate the method involved a similar quantitative experiment is described
below.
A 1.927g sample of lead shavings was
heated with an excess amount of sulfur. After the reaction was completed, 2.518
g of a lead and sulfur compound was obtained. Any sulfur that was not needed to
form the compound combined with the oxygen in the air to form sulfur dioxide, a
gas that escaped into the air. The following table summarizes the calculations
that were performed:
|
|
lead (Pb) |
sulfur(S) |
|
grams |
1.927 g |
2.518 g – 1.927
g = 0.591 g |
|
moles |
|
|
|
simplest whole
number ratio of moles |
|
|
|
ratio of atoms |
1 |
2 |
EMPIRICAL FORMULA- PbS2
Note that the simplest whole number
ratio of moles of the elements
in a compound is
the same as the ratio of their atoms in the empirical formula of that compound.
Ideally, the ratio of atoms should
have been 1.00: 2.00, but whole
numbers are not
usually obtained from an experiment.
Why?
Equipment
|
ring stand |
iron ring |
|
clay triangle |
crucible |
|
burner |
tongs |
|
ceramic/wire
gauze pad |
analytical
top-loader balance |
|
sandpaper |
deionized water
and dropper |
|
|
|
Chemicals
Mg
(magnesium ribbon), product magnesium
oxide
Procedure
1. Obtain
a clean, dry crucible (check for any cracks) and a cover. Weigh the crucible with its cover on an
analytical balance to 0.001 g. Record the weight on the data sheet.
2. Measure
out 35 cm of magnesium ribbon. Rub the
magnesium ribbon with fine sandpaper to remove any oxide coating. Cut the
magnesium ribbon into small pieces and place them into the crucible. Weigh the magnesium, crucible and cover on
an analytical balance. Record the
weight to 0.001 g
Setup of Apparatus
3. Heat the covered crucible gently, at first,
and then more strongly. The bottom of the crucible should glow red when it is
heated strongly. Periodically lift the
cover slightly with the crucible tongs to let air (O2, N2)
in. Replace the cover when the
magnesium begins to glow brightly.
Continue to heat the crucible strongly, frequently lifting the cover,
until only a white powder remains. You
should not be able to see any shiny metallic surfaces. Some hot magnesium will react with nitrogen
in air to form magnesium nitride. Most
of the magnesium reacts with oxygen to form magnesium oxide.
4. Allow the crucible to cool for 1 to 2
minutes and then add 7 or 8 drops of deionized water. Reheat the covered crucible.
The heating must be done gently, at first, or else the crucible will
crack and you will have to start from the beginning. Finish by heating strongly for 5 minutes. The magnesium oxide will react with water to
form magnesium hydroxide. The magnesium
nitride will react with water to form ammonia gas and magnesium hydroxide. The
ammonia gas will bubble off and just leave magnesium hydroxide. The magnesium hydroxide will decompose upon
heating to magnesium oxide and water vapor.
So eventually all the original magnesium ends up as magnesium oxide.
5. Let the crucible cool to room temperature. Weigh it with its cover on the analytical
balance to 0.001g. Again, you want to
use the same balance that you used for your other weighings. Reheat the
crucible for 5 minutes, cool and reweigh.
If the second weight is not the very close to the first weight, reheat
until a constant weight is obtained.
When the weight is constant, record it.
6.
Using the weight of magnesium ribbon form step 2. Calculate the mass of magnesium oxide that should have been
produced (theoretical yield). Now
determine from step 5 the mass of magnesium oxide that was actually produced. Use
these two values to calculate the percent yield for your magnesium oxide. Remember to show your work.
7. Look at steps 3 and 4. A total of 5 chemical reactions occurred on
the production of magnesium oxide.
Write balanced equations for all five reactions.
Disposal: Scrape the magnesium oxide product into the trash. Do not wash out the container. Any unheated magnesium should be returned to the setup tray. (Spill/Disposal: A)