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Lab3.1 – Empirical Formula Determination
***On lab day there is another assignment: Ch3 HW#3 Rev Worksheet on % Mass
and Empirical Formulas**
Pre-lab Questions
1. Read the lab completely. This ensures an efficient and safe experiment.
2. Based on your expertise in formula writing, what is your prediction for
the formula for magnesium oxide?
3. The manual from which this experiment was reproduced requires a lid over
the crucible, which makes it harder to see
the reaction, in fact it slows the reaction so much that the magnesium rarely
glows bright.
Why do you think this happens?
4. Without a lid, the reaction sometimes occurs so fast, it ‘spatters’ out of
the crucible. What effect do you think this
would have on your final mass of magnesium oxide?
5. Your teacher may come to your station and put forceps on the Mg to press
it down against the bottom of the
crucible. If any reaction has occurred, some grayish/white residue may
accompany his forceps out of the crucible.
What effect will this have on the final mass of the magnesium oxide?
6. If your mass of magnesium oxide produced comes out lower than expected,
what effect will this have on the mole ratio
between Mg and O?
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. Consider the following examples.
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 the 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 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 formula. In fact, it has been shown that individual hydrogen
peroxide molecules contain 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 C 2H2,
while that of benzene is C6H6.
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
determine the mass of oxygen used. The ratio between the number of moles of
magnesium used and the number of moles
of oxygen can then be calculated and the empirical formula of magnesium oxide
can be written on the basis of this ratio.
Objective
1. To determine empirical formula magnesium oxide.
Safety
1. Wear safety goggles.
2. Handle crucible only with tongs. There is a significant burn hazard
associated with handling of crucibles
because a hot crucible looks exactly like a cold crucible.
3. Remove the gas burner from beneath the crucible before using the crucible
tongs to remove the crucible.
Use forceps to grasp the rim of the crucible. Recall teacher demonstration.
4. If you make any adjustments during the lab, including adjusting the height
of the ring support, do so carefully.
The crucible may fall from the triangle when you move the ring.
5. Do not look directly at burning magnesium. The intense light given off in
the reaction may harm your eyes.
6. Do not inhale the smoke produced when the magnesium is burned. Keep your
face at arm’s length from the crucible.
7. Do not stir or otherwise disturb the product in the crucible. Dump the
contents in the waste beaker once it has cooled.
Procedure
All data, analysis, and conclusions should be copied to a separate piece of
paper. A template is found with the lab online.
1. Determine the mass of the empty crucible to the nearest 0.01g. Record the
mass in the data table.
2. Coil a 25cm length of magnesium ribbon and place it in the bottom of the
crucible. Try not to handle the magnesium.
Oils from your fingers will affect the mass, but at the same time, the Mg
needs to be tightly coiled to
ensure an exciting reaction.
3. Determine the combined mass of the crucible and the magnesium.
Record this mass in the data table.
4. Place the crucible on a clay triangle balanced on a ring support
clamped to a ring stand, as shown in the figure.
The figure shows a lid on the crucible. We will skip use of a lid.
5. Light the gas burner and adjust it to give a clear blue flame.
Place the burner under the crucible.
If necessary, adjust the height of the ring support so that the bottom of
the crucible is in the hottest part of the flame. The magnesium may begin
to glow really bright white during the reaction.
CAUTION: Do not look directly at the burning magnesium. The intense light may
hurt your eyes.
CAUTION: Be careful to keep the crucible at arm’s length at all times. Do not
inhale the “smoke” produced.
6. When the reaction is completed, the magnesium should be wholly converted
to a light grey powder, magnesium oxide.
If ribbon-like material remains in the crucible, heat the crucible and its
contents, checking every 2 or 3 minutes.
7. Allow the crucible to cool completely (at least 10 minutes) before
performing another weighing to determine
the combined mass of the crucible and magnesium oxide. Record your results.
8. Discard the magnesium oxide into the waste beaker at the teacher’s desk.
If time permits, repeat the experiment.
Data Table
Material
Mass
Empty crucible
Crucible and Mg (before heating)
Crucible and combustion product, MgxOy
Data Analysis
1. Determine the mass of magnesium ribbon used in the experiment.
2. Determine the number of moles of magnesium ribbon used.
3. Determine the mass of magnesium oxide that was formed.
4. Determine the mass of oxygen that combined with the magnesium. (See
Introduction.)
5. Determine the number of moles of oxygen atoms that were used.
6. Calculated the ratio between moles of magnesium used and moles of oxygen
used.
Express this ratio in simplest whole number form.
7. Give the empirical formula for magnesium oxide that is indicated by your
experimental data.
Results and Conclusions
1. Calculate the percentage error in your determination of the
magnesium:oxygen mole ratio.
Research online the accepted value. Yes, you may ask your teacher as well.
2. What are the major sources of your error in this experiment?
3. If some magnesium oxide was lost as smoke during the heating, how would
the magnesium:oxygen ratio
have been affected?
4. The lab required a strong, blue flame. A yellow flame will deposit soot on
the crucible. Why would this be a problem?
Going Further
1. When a mixture of hydrogen and oxygen is ignited by a spark, the gases
combine with an explosive release of heat
to give the product, water. Design an experiment that would allow you to
determine the empirical formula of water.
Name ___________________
Lab3.1 Empirical Formula Determination
Pre-lab Questions
2.
3.
4.
5.
6.
Objectives:
Procedure:
Data Table
Material
Empty crucible
Crucible and Mg (before heating)
Crucible and combustion product, MgxOy
Mass
Data Analysis (Show your work for credit.)
1. Mass of magnesium ribbon
= __________ g
2. Moles of magnesium
= __________ mol
3. Mass of magnesium oxide
= __________ g
4. Mass of oxygen that combined with the magnesium
= __________ g
5. Moles of oxygen
= __________ mol
6. Mole ratio between moles of magnesium and moles of oxygen
7. Empirical formula for magnesium oxide:
Conclusions
1.
2.
3.
4.
Ratio:
Download
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Lab3.1 Empirical Formulas.doc
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