2. Determination of the Empirical
Formula
• Determination of the Empirical Formula of
Magnesium Oxide
• Thermal decomposition of calcium carbonate
• Determination of the empirical formula of an
oxide of tin,

3. Agenda
• Must go over safety
• 3 labs-2 trials each
• Give back exam
• Next exam in 3 weeks!!!

4. Test taking
• An exam in a subject is not a measure of
weather you are a good person or not-
• It is how you prepared

5. Mg Lab

6. Determination of the Empirical
Formula of
Magnesium Oxide
• Determination of the Empirical Formula of
• Magnesium Oxide

7. Caution!!!!
• Eye protection is essential.
• Open flame will be present.
• Do not breathe the fumes generated.
• Once any burner is lit, assume ALL equipment is hot.
• Do not touch the crucible, lid, triangle, ring, or stand
during or after they have been heated.

8. Donot burn Mg
• If this happens . . . Look away

9. You will be asked to put glasses on
by me! . .

10. 2 main laws
This lab illustrates (i) the law of conservation of
mass and (ii) the law of constant composition.
• (i) The total mass of the products of a reaction
must equal the total mass of the reactants
• (ii) Any portion of a compound will have the
same ratio of masses as the elements in the
compound

11. GOAL AND OVERVIEW
• The quantitative stoichiometric relationships
governing mass and amount will be studied
using the combustion reaction of magnesium
metal.
• Magnesium is reacted with oxygen from the
air in a crucible, and the masses before and
after the oxidation are measured.

12. 78 % Nitrogen -20% oxygen
• Magnesium reacts vigorously when heated in the
presence of air. The Mg-O2 reaction is energetic
enough to allow some Mg to react with gaseous N2.
• Although there is a higher percentage of N2 gas in
the atmosphere than O2, O2 is more reactive and the
magnesium oxide forms in a greater amount

13. GOAL AND OVERVIEW
The resulting masses are used to calculate the
experimental empirical formula of magnesium
oxide, which is then compared to the theoretical
empirical formula. A crucible and Bunsen burner
will be used to heat magnesium metal to
burning.

14. Objectives of the data analysis:
1. Determine the expected formula for the ionic
oxide expected when Mg reacts with O2
2. Find the theoretical and actual yields of
MgxOy
3. Evaluate results using stoichiometry and
error analysis

15. Math mathematical operations
For water
2 H= 2 X 1.008 amu= 2.016 amu
1 O = 15.9994 amu= + 15.9994 amu
18.02 amu
MUST DO THIS FIRST!!!!
(18.02 amu )(6.022 x 10-23)(1.660× 10-24grams) =
= 18.02 grams = 1 mole H2O

16. What you will Not do!!!!

17. Can answer . . .
• If 10.00 grams of magnesium is burned in the
presence of oxygen-
• How much magnesium Oxide is produced??
2Mg(s) + O2(g) ——> 2MgO(s)

18. how many moles
• Determine how many moles are given in the
problem.
• Calculate the molar mass of the substance.
• Multiply step one by step two.
• Make sure you have a periodic table and a
calculator handy.

19. Need to get to molecular weight of
Mg```
1 Mg= 1 X Atomic Mass: ===== 24.305 amu=
This is to say Mg exist as 24.30 grams/Mol
*can be used as an conversion factor

20. Firstly . . We need
24.30 grams = 1 mole Mg
1 mole Mg = 24.3 grams
1 DOZEN = 12 THINGS!!!
(18.02 grams) = (1 mole H2O ) = 1
(1 mole H2O) (18.02 grams)

21. 40.3 grams = 1 mole MgO
1 Mg= 1 X Atomic Mass: ===== 24.305 amu=
1 O = 15.9994 amu= + 15.9994 amu
40.3 amu
MUST DO THIS FIRST!!!!
(18.02 amu )(6.022 x 10-23)(1.660× 10-24grams) =
= 40.3 grams = 1 mole MgO

22. Grams A = Moles A = Moles B = Grams B
• Grams to moles for Mg-(periodic table) And
• Moles of Mg to moles of MgO (the reaction)
• But –in total . . .need . . . . . .
• Grams A = Moles A = Moles B = Grams B
(Mg) (MgO)

23. .
2Mg(s) + O2(g) ——> 2MgO(s)
This says:
2 mol Mg = 2 mol of MgO
2 mol MgO = 2 mol of Mg
(2 mole Mg) = (2 mole MgO ) = 1
(2 mole MgO) (2 mol Mg)

24. Grams A = Moles A = Moles B = Grams B
10 g of Mg (1 mole Mg) (2 mole MgO ) = 0.823 mol of MgO
(24.3 g of Mg) (2 mol Mg)
0.823 mol of MgO (40.3 gram MgO) = 32.92 gram MgO
(1 mol of MgO)

25. EXPERIMENTAL PROCEDURE
• 1) Fire the empty crucible and lid for about 3
minutes to remove water, oils, or other
contaminants and to make sure there are no
cracks. The bottom of the crucible should glow
red-hot for about 20 seconds. Remove the
flame and cool the crucible with lid.

26. EXPERIMENTAL PROCEDURE
• 2) Record the mass of crucible and lid once it
has cooled. Do not handle it with your hands.

27. EXPERIMENTAL PROCEDURE
• 3) Obtain about 0.3 g (35 cm) magnesium
ribbon (do not handle the ribbon with your
hands). Fold the ribbon to fit into the bottom
of the crucible.
• 4) Record the mass of the magnesium ribbon,
lid and crucible.

28. EXPERIMENTAL PROCEDURE
• 5) Place the crucible securely on the clay
triangle. Set the lid slightly off-center on the
crucible to allow air to enter but to prevent
the magnesium oxide from escaping.

29. EXPERIMENTAL PROCEDURE
• 6) Place the Bunsen burner under the crucible,
light it, and brush the bottom of the crucible
with the flame for about 1 minute; then, place
the burner under the crucible and heat
strongly.

30. EXPERIMENTAL PROCEDURE
• 7) Heat until all the magnesium turns into
gray-white powder (probably around 10
minutes).
• 8) Stop heating and allow the crucible, lid and
contents to cool.

31. EXPERIMENTAL PROCEDURE
• 9) Add about 1 ml (~10 drops) of distilled
water directly to the solid powder. Carefully
wait for some of the gas that is generated
toward your nose, but be very careful . Record
any odor.
• 10) Heat the crucible and contents, with the
lid slightly ajar, gently for about 2 minutes and
then strongly for about another 3 to 5
minutes.

32. EXPERIMENTAL PROCEDURE
• 11) Allow the crucible to cool and then record
the mass of the crucible, lid and contents.

33. Please report ----DATA ANALYSIS
1. The mass of Mg metal used
2. theoretical yield of MgO from reaction:
Mg(s) + ½ O2(g)  MgO(s)
3. mass of oxide product formed
4. mass of O incorporated
(by difference; see eq. 4)

34. Please report ----DATA ANALYSIS
5. empirical formula of the oxide
6. percent by mass of Mg and O in the oxide
7. calculate the percent yield

35. 2 more experiments
• Thermal decomposition of calcium carbonate
• Determination of the empirical formula of an
oxide of tin,