1. Exp 3A: The Empirical Formula of an Oxide
Introduction
Compounds are pure substances
a combination two or more elements that form a new compound
Chemical Formula
a combination of symbols of the various elements that make up the compound
Formula unit
the smallest collection of atoms that provides information on a compound
the identity of the atoms
the relative number of each type of atom

2. Exp 3A: The Empirical Formula of an Oxide
Law of Definite or Constant Composition
A pure compound contains 2 or more atoms in definite proportions
No matter what its source, a specific compound is composed of the same elements in the same mass fraction or ratio
Every chemical compound has a formula that describes the type and number of different atoms in the compound
Analysis gives you the composition of AB2C
mass percentage
mass % A = (mass of A in the whole amount/mass of the whole amount) x 100%
molar composition or mole percent
mole % of A = (moles of A/total number of moles) x 100%

3. Exp 3A: The Empirical Formula of an Oxide
Question
What is a mole and what is a molar mass?
What are the molar masses of magnesium and atomic oxygen?
Answer
A mole is the quantity of a substance that contains as many molecules or formula units as the number of atoms in exactly 12 g of carbon-12
A molar mass is the mass of one mole
The molar masses of magnesium and oxygen are g and g, respectively

4. Exp 3A: The Empirical Formula of an Oxide
Mole
Chemical counting unit
1 atom Al = amu 1 mol Al = g
1 atom O = amu 1 mol O = g
1 molecule O2 = 32 amu 1 mol O2 = g
1 atom Na = amu 1 mol Na = g
1 atom Cl = amu 1 mol Cl = g
1 molecule Cl2 = amu 1 mol Cl2 = g
1 formula unit NaCl = amu 1 mol NaCl = g

5. Exp 3A: The Empirical Formula of an Oxide
Purpose
Observe the reaction of magnesium with oxygen
Determine the empirical formula of the product, magnesium oxide
Background
Oxygen (O2) is very reactive when heated
Many elements react with oxygen, forming an “oxide”
Nitrogen (N2) is very unreactive, even at high temperature
Only very active metals react with N2, forming a “nitride”
Water (H2O) and nitrides react to form “hydroxides” (compounds of a metal and a hydroxide, OH) and ammonia, NH3.
Heating the hydroxide converts it to an oxide and water vapor

6. Exp 3A: The Empirical Formula of an Oxide
Background
Final product is magnesium oxide
Primary reaction
Reaction between Mg and O2  MgO
Secondary reaction
- Reaction between Mg and N2  Mg3N2
- Mg3N2 and water (H2O) form Mg(OH)2
- Heating Mg(OH)2 results in formation of MgO and H2O
All Mg is now converted to MgO
Mass of O2 that reacted with Mg can be determined from the original mass of Mg and the mass of the final product, MgO
Laws of conservation of mass and number of moles are used to calculate this amount

7. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Prepare
Wash and dry a crucible with lid
Put the crucible on a clay triangle over a Bunsen burner and heat for about 3 min at high heat
Cool for 5-10 min (why?) and weigh the crucible with lid
Repeat heating, cooling and weighing until 2 consecutive weights are within g of each other
Record in your lab notebook in 4 decimals
Weigh out about 0.2 g Mg ribbon (record weight)
Fold Mg ribbon loosely (not too tight!) and put in crucible
Weigh crucible, Mg ribbon and lid. Record weight

8. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Heating
Put crucible, Mg ribbon and lid on clay triangle. Cover crucible with lid.
“Brush” bottom for 2-3 min with hot flame
Put burner under crucible and heat for 3 more min in the hottest part of the flame
Lift lid slightly with tongs to allow air to enter
Don’t open too far, because Mg will catch fire
Metal should glow bright-red
Repeat approximately every 3-5 min until no metal is visible anymore
all is converted to magnesium oxide powder
no glowing is visible anymore
Allow crucible to cool

9. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Analysis
Remove lid, put few drops of H2O in the crucible
What do you smell? Report!!
Cover the crucible. Brush the crucible again with the flame until the contents is dry
Heat on a hot flame for 8-10 min (why?)
Allow crucible to cool down to the point where it is close to room temperature
you feel no heat when you bring your finger within ½ in of the crucible
Weigh the crucible + content + lid. Record the weight
Heat again for 3 min
Cool crucible and obtain weight; record weight in 4 decimals
Repeat until weight is constant
2 consecutive weightings within g of each other

10. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Results
(1) Mass of empty crucible + lid (g, 4 decimals) ______________
______________
(2) Average mass of empty crucible + lid (g) ______________
(3) Mass of crucible, Mg, and lid (g) ______________
(4) Mass of Mg [= (3) – (2)] (g) ______________
After heating
(5) Mass of crucible, lid and oxide (g) ______________
(6) Average mass (g) ______________
(7) Average mass of empty crucible + lid [=(2)] (g) ______________
(8) Mass of oxide (g) ______________

11. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Calculations
Mass of O = mass of MgO – mass of Mg __________
Moles of Mg = mass of Mg/molar mass of Mg __________
Moles of O = mass of O/molar mass of O __________
Molar ratio of Mg:O = __________
Empirical formula for magnesium oxide =

12. Exp 3A: The Empirical Formula of an Oxide Experimental Procedure
Balanced equations
Mg with molecular oxygen
Mg + O2 
Mg with molecular nitrogen
Mg + N2 
Magnesium nitride with water
MgyNz + H2O Mg(OH)x
Heating magnesium hydroxide
Mg(OH)X + heat  Mg-oxide + H2O

13. Exp 3A: The Empirical Formula of an Oxide Chemicals
Post-Lab Assignment
Exp 3A: The Empirical Formula of an Oxide
Fill in the Results sheet (p. 65)
Show all your calculations. Calculations need to be complete and logically organized and written
Analysis
Determine the empirical formula for the oxide of magnesium.
Think carefully about the precision of the masses that you determined on the electronic balance. How many significant figures are justified in your answer?
Answer questions 1 and 2 on Post-Lab Assignments sheet

14. Exp 3A: The Empirical Formula of an Oxide Chemicals
Due Next Week
Exp. 3A:
Data/results Sheet
Post-lab Questions
Exp. 3B: Hydrates and their Thermal Decomposition
Prelab assignments 1 - 4
Read and understand
Goal of experiment
Procedure
Properties of BaCl2, CrCl3, CoCl2, NiSO4, CuSO4 and the hydrates mentioned in the lab text