Experiment
4-1 Determination of the Empirical Formula of a
Compound (4)
Lab Report = 90
(5) Introduction:
In this experiment, a measured mass of magnesium
metal will react completely to form magnesium
chloride. After the magnesium chloride is isolated,
the experimental percent magnesium chloride will be
calculated and compared to the theoretical value of
25.5%.
Whenever an experiment is carried out, an analysis of
the sources of error will help determine the accuracy
of the experiment. The theoretical percent of an
element in a compound can be calculated from its
formula. For example, the percent aluminum oxide,
Al2O3, is found by dividing two times the atomic mass
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of aluminum, 54.0, by the formula mass Al2O3, 102.0,
and multiplying by one hundred. The result is 52.9%.
To obtain good results in this experiment, it is
important to remove the water of hydration from the
solid, dry magnesium chloride. Repeated moderate
hearing over a burner flame, followed by cooling and
massing, will result in a constant mass. IF the
product is underheated, some of the water of
hydration will remain. If the product is overheated,
some of the magnesium chloride may decompose or
vaporize.
(2) Objectives:
1. Determine the mass of magnesium chloride
produced when a given mass of magnesium is
reacted with excess HCl.
2. Calculate the empirical formula for magnesium
chloride.
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3. Compare the empirical formula obtained
experimentally with the accepted formula.
4. Analyze the cause of any unexpected
experimental results.
(5) Apparatus:
centigram balance
hot plate
50-mL evaporating dish
wire gauze
50-mL beaker
Tirrill burner
ring stand and ring
beaker tongs
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safety goggles
laboratory apron
fume hood
Reagents:
3.0 M HCl
magnesium metal ribbon
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6.0 M HCl
Procedure:
Part 1
1. Put on your laboratory apron and safety goggles.
2. Measure the mass (0.0001 g) of a clean, dry 100
mL beaker labeled with an ID. Record the mass
and ID number in your data table.
3. Measure the mass of a strip of magnesium
ribbon (10-15 cm) and record it in the data table.
Place the magnesium in the beaker.
CAUTION: 3.0 M HCl and 6.0 M HCl are corrosive.
Wash spills and splashes off your skin and
clothing immediately using plenty of water and
baking soda. Notify your teacher.
4. Place the beaker in the operating fume hood.
Slowly add1-2 mL of 3.0 M HCl to the
magnesium in the beaker, immediately cover with
a watch glass to prevent loss of the spatter. wait
for effervescing to subside, add another 1-2 mL,
Experiment 4-1
wait, etc., until you have added a total of about
10-15 mL of 3.0 M HCl. Record your
observations in the data table.
5. If magnesium remains, add 1-2 mL of 6.0 M
hydrochloric acid slowly. Repeat until no more
magnesium remains unreacted. The HCl should
be in excess. Rinse the watch glass into the
beaker with DI water.
6. Put the uncovered beaker on a hot plate in the
fume hood, turn hotplate up to “2”, and allow the
product to dry.
7. Before leaving the laboratory, clean up all
materials and wash your hands thoroughly.
8. At the end of the period, place the beaker into
the drying oven to let dry over night.
Part 2 (after drying in drying oven)
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9. Put on your laboratory apron and safety goggles.
13. Measure the mass of the beaker and
magnesium chloride.
10. Record your observations of the beaker
contents in your notebook.
14. Dispose of the magnesium chloride by
dissolving it and washing the solution down the
drain.
11. Measure the mass of the beaker and
magnesium chloride. Record the data in the data
table.
15. Record your data on the class data table.
12. Set up a ring stand and wire gauze. Using a
Tirrill burner heat the beaker and contents until
no liquid is apparent. Allow the beaker to cool
unitl room temperature about 15 mins.
16. Before leaving the laboratory, wash your hands
thoroughly.
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(22) Data and Observations:
Identification number of beaker
Mass of beaker
Mass of magnesium
Mass of beaker and magnesium chloride after hot plate
Mass of beaker and magnesium chloride after heating
Observations while adding HCl:
Observations of the product after drying overnight:
(10) Experiment 4.1 Class Data
Period
--------
Team
2
3
4
5
6
7
8
9
---------
Mass Mg (g)
Mass MgCl2 (g)
-----------------
Average
% Mg
Deviation
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Experiment 4-1
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(26) Calculations:
Show all derivations!
(2) 1. How much magnesium reacted?
(2) 7. Use the results of calculations 1 and 3 to
determine the percent magnesium in
magnesium chloride.
(2) 2. Calculate the moles of magnesium that
reacted.
(2) 8. Calculate the class average percent
magnesium in the product.
(2) 3. Calculate the mass of magnesium chloride
produced.
(4) 9. Calculate the deviation of each determination
of the percent magnesium from the average.
(Deviation is basically how much the data is
off from the average.) Show one derivation,
not all.
(2) 4. How many grams of chlorine reacted?
(2) 5. Calculated the number of moles of chlorine
that reacted.
(2) 6. Use the results of Calculations 2 and 5 to
determine the ratio of chlorine atoms to
magnesium atoms in magnesium chloride,
and write the formula for magnesium
chloride.
(2) 10. Calculate the average deviation.
(2) 11. Calculate the percent error for your team
value.
(2) 12. Calculate the percent error for the class
average value.
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(16) Analysis:
(4) 1. Explain why an experimental error is more
likely to by made in the mass of the
magnesium chloride than the mass of the
magnesium.
(4) 2. Compare your experimental percent
magnesium to the theoretical percent
calculated in Prelab Question 4. Suggest a
cause for any experimental error.
Experiment 4-1
(2) 3. Compare the class results to the theoretical
results. Are the errors random or
systematic?
(2) 4. What is the maximum percent yield possible in
a reaction?
(4) 5. Suggest ways this experiment could be
improved.
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