Law of Definite Composition (MgO) Lab
Objective: Compounds always form with definite composition. In this lab, you will
confirm the chemical composition of the compound Magnesium Oxide.
Safety: Wear Goggles! Hot Crucibles look like cold Crucibles. Mg on fire can
cause serious burns. Do not pick it up with fingers.
Do not put hot Crucibles on paper or in trash can. Remember fires start at 451oF
Do not put the rubber hose of the Bunsen burner on too tight on the gas jet. They
are hard to remove if you really shove them on there.
Move all paper away from the Bunsen burner before lighting it.
Do not play with the lighters! Use them for lighting the burners and that’s IT!
Procedure:
1. Roll the piece of Mg ribbon you get from the supply table into a loose coil.
2. Find the mass of the crucible and its cover. Record as Mass of Crucible and Cover
3. Drop the piece of Mg ribbon into the crucible on the balance. Find the mass of the
crucible, the cover and the Mg. Record this on your data table.
4. Assemble the ringstand/clay triangle/bunsen burner/crucible as directed.
5. Place the Mg ribbon into the crucible.
6. Carefully begin to heat the bottom of the crucible (the lid should be on the crucible).
7. CAUTION: if a lot of smoke starts to appear, reduce or remove the flame.
8. After about 5 minutes of heating lightly, turn up the bunsen burner until the bottom of
the crucible gets red-hot. Do this for about 4 minutes.
9. Use the tongs to remove the lid of the crucible and heat it for another 4 minutes. Your
Mg should start to burn soon and turn bright white. If it doesn’t, call me over to your
table and I will try to “persuade” it to start on fire.
10. After it has burned up, turn off the bunsen burner and put the lid back on the
crucible. Allow the crucible and lid to cool. AFTER COOLING - find the mass of the
After heated mass of crucible, cover and magnesium oxide product.
11. Subtract the Mass of Crucible and Cover from the After heated mass of crucible,
cover and magnesium oxide product. Record this as the mass of the magnesium
oxide product.
12. If there is enough time left in the period, add about 10 drops of distilled water to the
crucible and magnesium oxide product. Put the lid back on the crucible and begin to heat
it again until you have driven off all of the water.
13. ALLOW IT TO COOL - then find the mass of the crucible, cover and magnesium
oxide product. Compare this mass to the After heated mass of crucible, cover and
magnesium oxide product you got in step 10. If your answers are within 0.03 grams of
each other, then you are done. If not, continue to heat your crucible until the answers are
within that range.
Data Table:
Mass of Crucible and Cover ___________g
mass of the crucible, the cover and the Mg ___________g
After heated mass of crucible, cover and magnesium oxide product _______g
mass of the magnesium oxide product ___________g
Calculations/Questions:
1. What was the mass of the piece of Mg ribbon you used?
2. FACT: The only element which combined with the magnesium when it was heated
was oxygen. QUESTION: Can you figure out what mass of oxygen must have
combined with the Mg?
3. Divide the mass of the original piece of Mg ribbon (from question #1) by the total
mass of the oxygen you found in question #2. (This is the ratio of Mg:O to be found in
magnesium oxide.)
4. FACT: Your answer to question #3 tells you how many grams of Mg can be found for
every ONE gram of Oxygen in Magnesium Oxide. QUESTION: Can you figure out
how many grams of oxygen would be in Magnesium Oxide if there was ONE gram of
Magnesium? (HINT #1: The answer is less than 1 gram. HINT #2: Think Bubble Lab
when you were asked to find the number of centimeters in ONE inch and then the number
of inches in ONE centimeter).
5. FACT: The accepted ratio of Mg:O is 1.52g Mg:1g O. QUESTION: Find the
percentage error of your result by doing the following calculation: (yes, those lines mean
absolute value) (HINT: Accepted ratio = 1.52)
Percentage error = Accepted ratio – the ratio you got  x 100
Accepted ratio
6. Use the accepted ratio of Mg:O to determine the number of grams of Mg which
would be required to react completely with 16.0 grams of O. (remember to use 3
significant digits in your answer). HINT: Do you remember how to do conversions?
7. Use the accepted ratio of Mg:O to determine the number of grams of O which would
be required to react completely with 42.0 grams of Mg. HINT: Do you remember how
to do conversions?
8. Look on the periodic table, what is the mass of Mg to three significant digits?
9.
Look on the periodic table, what is the mass of O to three significant digits?
10. Divide the periodic table mass of Mg by the periodic table mass of O. Isn’t this the
same ratio as given in #5?
11. The formula for Magnesium Chloride is MgCl2. What ratio do you think Mg:Cl will
have? (HINT: Use their mass numbers from the periodic table)
12. Write the formula for Water. What ratio do you think H:O will have?
What is due for this lab:
1. Data Table
2. Answer the questions (you don’t need to copy them)