Chemistry 140
Please have parts 1, 2, 3 and 4 (tables only) ready before class on
Wednesday, October 11. Write an abstract and paper-clip it to the front of
your individual writeup. The abstract and the copied pages of the notebook
are due in class on Wednesday, October 18.
This is the standard lab manual format for a lab. It will be your job to
translate it into the familiar of the report expected in Chem 140 (some
guidelines are given at the end of the lab protocol below).
Lab 2: Determination of the empirical formula of the product of magnesium heating
In this experiment, you will determine with which component of air (nitrogen or
oxygen) magnesium will react when heated, by identifying the percent composition of
the product of that reaction.
Air is composed of 78% nitrogen (in the form of N2) and 21% oxygen (in the form of O2),
with a trace of other gases. When a strip (or ribbon) of magnesium is heated in a
porcelain crucible, it emits an intense white light for a few minutes and a gray-white
powdery residue remains. The experiment will help you determine the identity of that
powdery residue.
One possibility is that magnesium metal simply oxidizes by heating it in air:
2 Mg (s) + O2 (g) ⇒ 2 MgO (s)
The other possibility is that magnesium reacts with the nitrogen in the air to form
magnesium nitride:
3 Mg (s) + N2 (g) ⇒ Mg3N2 (s)
By determining the % Mg in your product, you can identify the product as MgO or
Mg3N2. From the mass of magnesium before heating and the mass of the product, you
can calculate the experimentally determined percent Mg in the product:
exp erimental %Mg =
Materials:
!
Magnesium (Mg)
Crucible (with cover)
Bunsen burner
mass Mg
" 100
mass product
Matches
Tongs
Metal ring
Clay triangle
Wire gauze
Digital balance
Procedure: Work in groups of 2.
0. Figure out where the equipment for the lab is kept (some of it will be in the cabinet,
some of it will be on the cart).
1. Inspect the crucible for cracks. If your crucible is cracked, exchange it for a new one.
Clean the crucible and cover thoroughly. The crucible may be stained, so you will not
be able to get it completely white. After cleaning be sure to handle it ONLY with
metal tongs.
2. Place the clay triangle on an iron ring clamped to a ringstand. Using tongs, transfer
the crucible and its cover to the clay triangle. See if you can identify the materials list
above in the picture.
Use the striker to light the Bunsen burner.
Begin heating them with the Bunsen burner. Adjust the burner to obtain a blue flame
having an inner lighter blue cone. The tip of the cone is the hottest part of the flame; use
this part of the flame to heat the crucible red hot.
3. After heating the crucible, cool it in place on the ringstand for about five minutes to
room temperature.
4. Using the digital balance, weigh the crucible and cover to the nearest milligram
(.001g) and record the mass in your notebook. Place the magnesium sample into the
crucible and then weigh the crucible, cover and sample. Record this mass and the
physical appearance of the magnesium metal in your notebook. Notice that you can't
merely tare the crucible and cover.
5. Cover the crucible with its lid and place it inside the clay triangle supported by a ring
and ringstand (see demo model). Heat GENTLY. After 10 minutes check to make sure
that most of the metal has reacted by using tongs to lift the lid SLIGHTLY. If the metal
is burning, do not look directly at it, but replace the lid and continue heating gently
until flames are no longer visible.
6. Remove the lid and heat the crucible with the hottest part of the flame for 5 minutes.
Record the physical appearance of the residue.
7. When the crucible and its contents have cooled (after five minutes), weigh them to the
nearest milligram and record this mass in your notebook. Use the same digital balance
you used for the previous massing.
8. Dispose of the magnesium product in the container designated for waste disposal.
Clean the crucible and cover.
9. Repeat the experiment for a total of three trials.
Sample Data Sheet for magnesium experiment:
Trial # ____
Mass of empty crucible and cover
Mass of crucible, cover and Mg
Mass of Mg
Mass of crucible, cover, and product
Mass of product
___________________
___________________
___________________
___________________
___________________
Results:
theoretical % Mg in MgO
theoretical % Mg in Mg3N2
experimental % Mg in product
___________________
___________________
___________________
To calculate the theoretical % Mg in a compound, calculate the mass of Mg in one
formula weight of the compound, then divide by the formula weight (or molar mass)
of the compound.
% Mg =
mass of Mg in 1 formula weight of compound
" 100
formula weight of compound
And now for the hints on how to set up your lab book:
Your name, your partner’s name, date of experiment
!
Lab 2: Determination of the empirical formula of the product of
magnesium heating
Part 1. Purpose
Please write a sentence or two stating the goal of this experiment, based on the
information above.
Part 2. Materials and methods
Chemicals needed: magnesium metal strip
Examine the setup shown above, draw the equipment shown and label the various
pieces of equipment.
Part 3. Procedure
There should be no major modifications to the procedure above.
Dispose of any magnesium-containing materials in the container designated for
magnesium waste disposal in the hood.
Part 4. Original data
Perhaps the sample data sheet shown above could be converted into a table for the
three trials you will doing.
Part 5. Calculated results
Note that you should include your calculations for both of the theoretical magnesium
percent compositions as well as your experimental magnesium percent composition
(one trial’s
worth will do; I trust your other two trials; calculations will be done in an identical
manner!). Organize your calculated results into a table.
Part 6. Group results
Write all three of your experimental magnesium percent compositions on the board and
record all groups' experimental magnesium percent compositions. Calculate the mean
and standard deviation, and comment about where your results fit in (for instance, are
your results outliers?).
Part 7. Questions
1. Why can't you simply tare the crucible and cover when recording masses in step 4?
2. From your results, is your product MgO or Mg3N2? Looking over the results of your
three trials, did you have a systematic error? In any case, identify the sources of error in
your experiment and how you would expect the error to affect your results.
3. Compare the appearance of magnesium metal with the product after heating. In what
way could you tell that a chemical reaction occurred?
4. After looking at the group results from part 6, are you more or less confident of your
answer to question 2? Mention how the standard deviation can be used in this context.
Did the class as a whole suffer from a systematic error? Were there any outlier values
which, if thrown out, improve the standard deviation dramatically?
Part 8. Conclusion
The conclusion should be a brief recap of your results and sources of error (confidence
you have in your results).
First sentence: “We determined that the magnesium percent composition of our
material was _________, __________ , and __________ in three trials, which resulted in a
mean value of ____________ ± _____________ %.”
Second sentence: “From these results, we concluded the empirical formula of the
product of heating magnesium in air is ___________________ because
______________________________.”
Next couple of sentences: Mention any significant random and/or systematic errors and
how they could be fixed or not fixed next time.
Next sentence: “The class mean was _________ ± _________ %, which is
(consistent/inconsistent) with our result.”
Last sentences: Did you accomplish the goal of the experiment? How confident are you
of your results?
Abstract
Again, a short (less than 100 word) summary of the major result(s) of your experiment,
and the method by which you achieved this result.
Name, school affiliation, title, text as usual. Keep in mind: What was the major result
(and uncertainty)? By what method did you achieve the result?