EXPERIMENT
/
Stoichiometric Determinations
Soichiometrjc measurements are among
the most important in chemistry
indicat’
proportions by mass in which -various
substances react.
hc
Determination of a Formula
Introduction
Magnesium metal is a moderately reactive elementary substance. At room lcmpcraturc,
magnesium reacts only very slowly with oxygcti and can be kept for long periods of time
without appreciable oxide buildup.. At elevated LcmpraLures, however, magnesium,wifl.
‘ignite inan excss.of oxygen gas, burning with an intensely white flar:rre’ and producing.
magnesiuth oxide, Because of the brightness of its flame, magnesium is used in flares and
in photographic flashbulbs.
In thiscxpcrimcnt. however, you will be heating magnesium in a closed container
called a crucible, exposing it only gradually to the oxygen of the air. Under these condo
lions, the magnesium will undergo a more controlled oxidation, gradually turning from
shiny metal to eravish-white powdered oxide. Because the air also contains a great deal
of nitrogen gas, a portion of hc magnesium being heated may he converted to mag
nesium nitride, Mg,3N2, rather than magnesium oxide. Magnesium nitride will react with
water and, on careful heating, is converted into magnesium oxide
Mg3N2
+
0
2
3H
-‘
3MgO + 2NH
3
The ammonia produced by this reaction can be detected by its odor, which is
released on heating the mixture.
Magnesium is a Group hA metal, and its oxide should have the formula MgO.
Based on this formula, magnesium oxide should consist of approximately 60% mag.
nesium by weight. By comparing the weight of magnesium reacted, and the weight of
magnesium oxide that results from the reaction, this will be confirmed.
102
Experiment 9
Safety
Precautions
Stoichiometric Determinations
• Wear safety glasses at all times while in the laboratozy.
• Magnesium produces an mtenselywlute flame if ignited, wiuch may be damag
ent ignites in the crucible
lug to the eyes. If the magnesium used in this experim
. Do not look
heating
by aàcident, immediately cover the crucible and stop
sium nitride to mag
• When water is added to the crucible to convert magne
when heated. Use only
spatter
may
nesium oxide, the contents of the crucible
crucible strongly until
the
heat
not
gentle heating in evaporating the water. Do
nearly alL the water has been removed.
• Use crucible tongs to handle the hot crucible and cover.
If it is spilled, wash imme
• Hydrochloric acid is damaging to skin and clothing:
diately and inform the instructor.
Apparatus/Reagents Required
e, magnesium turnings (or rib
porcelain crucible and cover, crucible tongs, clay triangl
bon), pH paper, 6 M HCI
Procedure
in ink.
Record all data and observations directly in your notebook
e and cover are extremely Iragile
Obtain a crucible and ,cover and examine. The crucibl
and expensive. Use
caution
in handl.ing them.
it gently with a paper towel to
If there is any loose dirt in the crucible, moisten and rub
hood, add 5—10 mL of 6 Al
the
to
it
remove the dirt. If dirt remains in the crucible, bring
d the I-[Cl and rinse the
Discar
s.
I-{CI and allow the crucible to stand for 5 minute
t with the instructor
consul
crucible with water. If the crucible is not clean at this point,
crucible has been
the
After
e.
about other cleaning techniques, or replace the crucibl
cleaned, use tongs to handle the crucible and cover.
e and cover to the triangle.
Set up a clay triangle on a ring stand. Transfer the crucibl
arms can be bent slightly if
The crucible should sit firmly in the triangle (the triangle’s
necessary).
thy them. When the crucible
Begin heating the crucible and cover with a small flame to
flame to full intensity, and
and cover show no visible droplets of moisture, increase the
heat the crucible and cover for 5 minutes.
to room temperature.
Remove the flame, and allow the crucible and cover to cool
move them to a clean dry
When the crucible and cover are completely coo!, use tongs to
Experiment 9
Stoichiometric Determinations
103
watch glass or flat glass plate. Weigh the crucible and cover to the nearest milligram
(0.OOlg).
Return the crucible and cover to the day triangle. Reheat in the full heat of the burner
flfr5mnutes.Allowtheucibleicoverto cool complete!ytoroomtemperature.
Reweigh the crucible after it has cooled. If the weight this time differs from the earlier
weight by more than 5 mg (0.005 g), reheat the crucible for an additional 5 minutes and
reweigh when cool Continue the heating/weighing until the weight of the crucible and
cover is constant to within S mg.
Add approximately 1 teaspoon of magnesium turnings (or about 8 inches of magnesium
ribbon coiled into a spiral) to the crucible.
Using tongs, transfer the crucible/cover and magnesium to the balance and weigh to the
nearest mg (0.001 g).
Set up the crucible on the clay triangle with the cover very slightly ajar. (Sec Figure 9-1.)
With a very small flame, begin heating the crucible gently.
• Figure 9-1. Set-up for oxidation
of magnEsium.
If the crucible begins to smoke when heated, inuncdiatcl cover the magnesurn com
pletely and remove the heat for 2—3 minutes. The smoke consists of the magnesium oxide
product and must not be lost from the crucible.
Continue to heat gently for 5—10 minutes with the cover of the crucible slightly ajar.
Remove the heat and allow the crucible to cool for 1—2 minutes.
Remove the cover and examine the contents of the crucible. If portions of the mag
nesium still demonstrate the shiny appearance of the free metal, return the cover and
heat with a small flame for an additional 5 minutes; then reexamine the metal. Continue
heating with a small flame until no shiny metallic pieces are visible,
When the shiny magnesium metal appears to have been converted fully to the dull gray
oxide, return the cover to its slightly ajar position, and heat the crucible with the full heat
of the burner flame for 5 minutes. Then slide the cover to about the half-open position
and heat the crucible in the full heat of the burner flame for an additional 5 minutes.
Remove the heat and allow the crucible and contents to cool completely to room
104
Experiment 9
StoichiometriC Determinations
sheet of dean
the clay triangle and set it on a
temperature. Remove the crucible from
paper on the lab bench.
Rinse any
large chunks of solid in the crucible.
With a stirring rod, gently break up any
distilled
into the crucible with a few drops of
material that adheres to the stirring rod
spreading
drops of distilled water to the crucible,
water. With a dropper, add about 10
the water evenly throuszlt the sobd.
position.
and set the cover in the slightly ajar
Return the crucible to the clay triangle,
been
has
the crucible to drive off the water that
cover
With a very small flame, begin heating
the
close
the heating. Remove the flame and
added. Beware of spattering during
of the crucible if spattering occurs.
forceps) in the
piece of moistened pH paper (with
As the water is driven off hold a
reacted with
the crucible. Any nitrogen that bad
stream of steam being expelled from
a basic
during the heating and should give
the magnesium is driven off as ammonia
note the odor of ammonia).
response with pH paper (j’ou may also
so that it is in
has been driven off, slide the cover
When it is certain that all the water
Heat the
flame.
and increase the size of the
approximately the half-open position,
of the burner for 5 minutes.
crucible and contents in the full heat
When com
cool completely to room temperature.
Allow the crucible and contents to
g).
contents to the nearest milligram (0.001
pletely cool, weigh the crucible and
full heat of the
heat for another 5 minutes in the
Return the crucible to the triangle and
and reweigh.
cool completely to room temperature
burner flame. Allow the crucible to
that agree
weights
and contents should give
The two measurements of the crucible
additional
for
is not obtained, heat the crucible
within 5 mg (0.005 g). If this agreement
weighings agree within 5 mg.
5-minute periods until two successive
determination.
Clean out the crucible, and repeat the
of magnesium
that was taken, as well as the weight
Calculate the weight of magnesium
percentage of
the
completion of the reaction. Calculate
oxide that was present after the
mean for
your experimental data. Calculate the
magnesium in the magnesium oxide from
your two determinations.
oxide, and
f magnesium (by mass) in magnesium
Calculate the theoretical percentage
deter
your
in
error
value. Calculate the percent
compare this to the mean experimental
mination.
V
Experiment 9 Stoichiometnc Determinabons
fWQuS:
107
,,
DETERMINAT1ONOFAFORMULA
L
Calculate the percentage by mass of magnesium and oxygen nagnesium oxide, MgO.
2.
Suppose 2.033 g of magnesium is heated in air. What is the theoretical amount of magnesium oxide tlt
should be produced?
Section
Name
Date
it ksctor
FORMULA Of A COMPOUND
RESUIJS/OBSERVATIONS
Trial 1
Trial 2
Weight of empty crucible (after first heating)
Weight of empty crucible (after second heating)
Weight of crucible with Mg
Weight of Mg taken
Weight of crucible[MgO (after first heating)
Weight of crucible/MgO (after second heating)
Weight of MgO produced
Weight of oxyn gained
% magnesium in the oxide
Mean % maincsum
Error
113
114 Experiment 9
Report
QUESTIONS
L
If water had not been added to..ur initial product, what error in the percentage magne
sium deter
mined would have resulted (that is, if part of the product bad been magnesium nitride
)?
2.
If the magnesium oxide smoke had been lost in too great an amount during
the heating of the crucible.
would this have made the clculatcd % Mg in the product too high or too low?