Formula of Magnesium Oxide Lab !
2/11/11 Blue Chem Comm!
Name______________________
Partnerʼs Name______________________
The law of Constant Composition states that atoms combine in proportions to form compounds,
and the law of Multiple Proportions says atoms combine in whole number ratios. Today we will
use that property to determine the way in which magnesium combines with oxygen.
Purpose: Identify the empirical formula of magnesium oxide
Safety Issues: Because we will be burning a flammable substance that burns with a very hot
flame, you MUST have close toed shoes, and have your hair tied back.
Procedure
1. Set up the ring stand, clay triangle, and Bunsen burner as shown in class.
2. Wipe the inside of the crucible out with a damp paper towel and return it to the clay triangle.
3. Heat the empty crucible with a bunsen burner for 10 minutes. This burns out any impurities
that may interfere with your readings.
4. Allow the crucible to cool for 5 minutes and then record its mass.
5. Obtain a strip of magnesium metal. Using the steel wool provided, sand the strip until it is
shiny. (This will remove the coating of magnesium oxide).
6. Coil the strip of magnesium into a tight coil.
7. Transfer the coiled strip to your weighed crucible. Reweigh the crucible and record your
results (subtract the mass of the crucible to determine the mass of magnesium).
8. Heat the crucible and its contents with a moderately vigorous blue flame . The magnesium
will start to glow, but if it ignites quickly cover the crucible. (Use tongs or tweezers to handle
the cover!)
9. Continue to heat the crucible for 8-10 minutes, covering the crucible anytime it ignites too
vigorously.
10. When complete, turn off the burner and allow the crucible to cool to room temperature, about
5 minutes.
8. Since we are using air as our source of oxygen, we may form some magnesium nitride during
the experiment. (Remember that air is over 70% nitrogen). To convert the nitride to the
oxide, slowly add 10 drops of distilled water to the cool crucible using a dropper. Do not add
water to a hot crucible! While adding the water, note the odor of the gas being evolved. Don’t
“sniff” the gas—waft it carefully towards your nose.
9. Heat the crucible with a blue flame for an additional five minutes. Allow the crucible to cool
to room temperature for about 5 minutes and then take the mass (this is the mass of
magnesium oxide)
Table 1: Calculating the % mass in your experimental compound
Mass of
magnesium (g)
(Step 7)
mass of
magnesium oxide
(g)
(Step 9)
mass of oxygen
that combined with
magnesium (g)
% magnesium
% oxygen
magnesium
oxygen
magnesium oxide
magnesium oxide
You should now be able to figure out the mass of oxygen that combined with magnesium to form
magnesium oxide. However, we do not know the ratio of atoms of oxygen and magnesium that
joined together to form a molecule of magnesium oxide. Although there are an infinite number
of combinations, We will assume that the likely combinations are small numbers, and that there
are three possible combinations of magnesium and oxygen that could combine to form
magnesium oxide:
MgO, MgO2, Mg2O
Only one of these is correct. To determine the correct one, you need to determine the percent
composition of each of the three compounds, and see which one matches the data that you found.
Using the periodic table, find the atomic masses of Mg and of O. The units are amu (atomic
mass units). For each possible formula of magnesium oxide, calculate the % mass of magnesium
and oxygen. (note, % values do not have units -- the % is the unit)
Table 2: Calculating the % mass for your theoretical compounds
Formula
Total mass of
compound
Total mass of
oxygen in
compound
% composition
of oxygen
Total Mass of
magnesium
in compound
% composition of
magnesium
MgO
MgO2
Mg2O
Based on the data in table 1 and 2, decide which formula in table 2 is most likely the correct
formula of magnesium oxide. Once you have done this, calculate the percent error in your
experiment by comparing the % oxygen in your experiment to the % oxygen in the theoretical
compound that most closely matches your data in table 3.
Table 3: Calculating the % error in your experiment
% oxygen in your
experimental compound
% oxygen in the correct
theoretical compound
The formula of magnesium oxide is: ________________________
% Error
(experimental theoretical)/theoretical
Lab report writeup
Purpose
As written on lab report
Rationale
Explain how the experiment will allow you to determine the correct formula
for magnesium oxide.
Procedure
copy and paste from the lab (download from drbrown.info)
Data
This section should include all relevant data generated during your
experiment. It should be labeled to indicate what it is, and proper units should
be included. Relevant data is data that you either use in the calculations
section or refer to in the discussion section.
Calculations
This section should include all calculations you needed to do for your
experiment. Set up all equations, label them to show what they are
calculating, and include all units. This section should include the calculation
of masses of components, and the calculation of percent masses for both your
experimental compound, and your three possible sample compounds. You
also should include the calculation of the % error. You may hand write this
section if you wish.
Discussion
Include the following sections:
First sentence/
Conclusion
The purpose of this experiment was to figure out the empirical
formula for magnesium oxide. your first sentence should clearly state what
you decided the formula for magnesium oxide was.
Supporting
Evidence
Explain why the data from your experiment support your conclusion.
Use specific examples from the data, but remember anything that you
mention here, must be either in your data section or your calculations
section.
Theory
Agreement
The next part should explain why your conclusion also agrees with
what you know to be true about ions and ionic compounds. Using the
theory of why ions and ionic compounds form (to fill valence shells),
explain why the formula you state in the conclusion makes sense.
Error
Was the error in your experiment big or small? Do you think the error
prevented you from making a valid conclusion? Why or why not?
Assuming you did the experiment perfectly, what might be some sources of
error that would have affected your experiment?
Lab report Grading Rubric
Structure of Writeup
All sections required are present
Proper grammar and spelling were used throughout
Sufficient attention was paid to organization and neatness
Research Question (Purpose)
The research question accurately explains the purpose of the experiment
Rationale
The rationale is correct
The rationale explains how the procedure will answer the purpose
Procedure
The procedure is clearly and correctly copied
Data
0
0
The data is presented only once
The data is presented clearly in a form that best allows the reader to understand it
The level of error in the data shows an appropriate level of attention to detail and
procedure
The units for all data points are labeled correctly and clearly explained
Calculations
Calculations are complete
Calculations are correct
Calculations are labeled to explain the purpose of the calculation
All numerical values in the calculations have units
Calculations are broken into sufficient steps to show how the final value was calculated
Discussion
The writing in the discussion section is clear and easy to follow
Conclusion
The conclusion is drawn from the experiment
The argument made in the first sentence is based on a logical conclusion
The argument made in the first sentence connects to the research question
Supporting Evidence
0
0
The logic in the Evidence section is correct
The data cited in the Evidence section is correct and consistent with the information
contained in the Data section
Sufficient examples to support the conclusion are given
A thorough and expansive explanation is given to support the conclusion
Theory Agreement
The correct theory is used to explain the conclusion
The theory itself is clearly and correctly explained
How the data matches the theory is clearly and correctly explained
Error
The amount of impact of the error on the experiment is judged correctly
The logic in explaining the impact of the error is clear and correct
Sources of error are sufficient and correct