Name: ___________________________________________
Lab – Magnesium Oxide Empirical Formula
Date: _______________
Introduction
The specific elements involved in this lab activity are magnesium and oxygen. They will react to form
magnesium oxide, a stable ionic compound. According to the Law of Conservation of Mass, the following
word equation should make sense:
mass of magnesium + mass of oxygen = mass of magnesium oxide
Table 1 – Physical and Chemical Characteristics [complete the chart] (4 points)
Reactive
or
Stable?
Physical
State
(s/l/g)?
Electronegativity
Lewis
Dot
Diagram
Electron
Config.
Metal,
Nonmet.,
Cmpnd?
Symbol
Magnesium
Oxygen
Magnesium
Oxide
PROCEDURE - the safety and success of this procedure requires careful work! Read the directions carefully.
Wear safety goggles and use care around hot equipment - keep lab stations free of clutter and combustible
materials. Handle the crucible and its contents with great care.
1.
a.
b.
One member of the lab group should set up the equipment - Bunsen burner, ring, clay
triangle, crucible tongs (do not use plastic-coated beaker tongs or test tube holders). Position
the iron ring with clay triangle just above the top of the Bunsen burner – you will apply a
very hot flame on the bottom of the crucible, so they must be close.
Another group member should obtain and polish (use steel wool) a piece of magnesium
ribbon whose mass is approximately 0.10 g. It is important that the magnesium be cleaned
of any oxide coating - it will shine uniformly when this is done properly.
2.
Use steel wool to clean the inside of a crucible (do not use water), and then obtain the mass of the
crucible and lid. Record this mass in Table 2.
3.
Tear the piece of magnesium into small pieces and place them in the crucible - this will maximize the
surface area of the magnesium available to react during heating. Then, obtain and record the
combined mass of the crucible, lid, and contents.
4.
Place the crucible and contents on the clay triangle. Be sure that the equipment is level and that the
crucible rests securely on the triangle. Light the burner and center it under the crucible. Adjust the
burner to obtain a double-blue cone in contact with the bottom of the crucible. Once the
magnesium ignites, put the lid on the crucible. For best results the magnesium must burn very
slowly and the fine, white "smoke" should not be allowed to escape from the crucible.
5.
After a short interval (10 - 15 seconds), lift the cover and allow fresh air to enter the crucible. The
magnesium should re-ignite and you should replace the cover. Repeat this procedure (venting and
recovering the crucible) until the magnesium no longer burns brightly, at which point you should
Name: ___________________________________________
Lab – Magnesium Oxide Empirical Formula
Date: _______________
turn off the flame. Stir the contents gently with a glass stirring rod so that non-reacted
magnesium will be able to react completely.
6.
At this point adjust the lid so that the crucible is mostly covered but there is a gap allowing a steady
flow of air into the crucible. Heat the crucible with a hot flame for a few minutes. There should not be
any metallic-looking material left in the crucible, and the contents should be uniformly grayish-white.
7.
Turn off the flame and gently pulverize the contents of the crucible with a glass stirring rod - try to
avoid having the powder stick to the glass. Let the crucible and contents cool for about 5
minutes, then add 10 drops of water from a wash bottle to the crucible. Take note of any odor by
gently wafting the air above the crucible toward your nose. Heat the product until it is dry (do not
replace the cover).
8.
After the crucible and contents are cool, obtain and record the final mass of the crucible, lid, and
contents. Observe the magnesium oxide formed, and then dispose of the powder after wrapping it in
a damp paper towel. Complete the calculations and questions that follow.
Reference Table – Look up the following atomic masses: (1 point)
Magnesium:
______ g/mol
Oxygen:
______ g/mol
Magnesium oxide:
______ g/mol
Table 2 – Experimental Data – 4 points
1
2
3
4
5
6
7
8
Mass of crucible, cover and Mg
Mass of crucible and cover
Mass of Mg used
Mass of crucible, cover and product
Mass of product (magnesium oxide)
Mass of oxygen in compound
Moles of Mg used (before reaction)
Moles of Oxygen used (after reaction)
Ratio – Magnesium : Oxygen (rounded)
9 [divide both mole totals by the smaller of the two] – round to
the nearest non-zero integer or to a “__.5.”
Empirical formula of compound
10
[use answer from step 9] – this formula may not be correct.
Percentage of oxygen in compound by mass
11
[experimental] – use the measured masses in Lines 5 and 6
Percentage of magnesium in compound by mass
12
[experimental] – use the measured masses in Lines 3 and 5
Percentage of oxygen in compound by mass [theoretical]
13
– this is a percent composition problem – use the “real” formula
Percentage of magnesium in compound by mass
14 [theoretical] – this is a percent composition problem – use the
“real” formula
Percent error of magnesium
[using theoretical percent and exp. percent by formula]
Percent error of oxygen
16
[using theoretical percent and exp. percent by formula]
15
Name: ___________________________________________
Date: _______________
Lab – Magnesium Oxide Empirical Formula
Analysis Questions: Be sure to show all calculations and units! (2 points each)
1. Using laboratory data, calculate the percent composition of magnesium oxide. Use the
measured masses of magnesium, magnesium oxide, and oxygen.
2. Write the correct formula for magnesium oxide: ________________________
3. Compare the experimental percent of oxygen in the compound to the theoretical value by
calculating percent error.
4. Air is a mixture of several gases, including a large amount of nitrogen. When magnesium
burned in air, some magnesium nitride was formed. It is black while magnesium oxide is
white. Use your knowledge of ion charges to write the correct formula for magnesium
nitride. (In this lab, whatever magnesium nitride formed was converted to ammonia gas,
leaving more magnesium oxide, when you added water to the crucible. Smell the ammonia?)
5. Determine how many moles of magnesium and moles of oxygen reacted.
6. Find the ratio you got in your data table (Line #9). This whole number corresponds to
the subscript next to the larger quantity of moles (either Mg or O), indicating how many Mg
or O atoms there are in each formula unit. What is your formula? What should it be?
7. We can actually predict the expected amount of product that should have formed,
assuming a perfect outcome. Use the theoretical Mg percent composition from Table 2. If
you know the mass of magnesium you started with, and you know that all of that mass is
expected to end up in the magnesium oxide, simply use the percent composition you
calculated above to figure out the ideal mass of product. Use Mg% as a decimal:
mass of Mg used ÷ Mg% = mass of MgO expected
8. How well does your data support the Law of Conservation of Mass? You can express the
“yield” as a percent, much like percent error. This will indicate how much of the total you
got:
% Yield =
Experimental Yield (Chart #5)____ * 100
Theoretical Yield (Question #7)