Exp 7: Empirical Formulas
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
1. the identity of the atoms
2. the relative number of each type of atom
Exp 7: 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
• Interpretation of a Chemical Formula:
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By pieces of matter: atoms, molecules, ions
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By the number of moles: where a mole is a “bunch” of pieces of matter
• 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%
Exp 7: 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
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A mole is a quantity of substance whose mass in grams is numerically
equal to its mass in amu
•
A molar mass is the mass in grams of one mole of a substance
•
The molar masses of magnesium and oxygen are 24.305 g and 15.9994
g, respectively
Exp 7: The Empirical Formula of an Oxide
Mole is also considered a Chemical counting unit
1 atom Al = 26.98 amu
1 mol Al = 26.98 g
1 atom O = 16.00 amu
1 mol O = 16.00 g
1 molecule O2 = 32 amu
1 mol O2 = 32.00 g
1 atom Na = 22.99 amu
1 mol Na = 22.99 g
1 atom Cl = 35.45 amu
1 mol Cl = 35.45 g
1 molecule Cl2 = 70.90 amu
1 mol Cl2 = 70.90 g
1 formula unit NaCl = 58.45 amu 1 mol NaCl = 58.45 g
Exp 7: The Empirical Formula of an Oxide
NOTES for the EXPERIMENT
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
Exp 7: The Empirical Formula of an Oxide
Background
•
Final product is magnesium oxide
1.
Primary reaction
Reaction between Mg and O2  MgO
2.
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
3.
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
4.
Laws of conservation of mass and number of moles are
used to calculate this amount
Exp 7: The Empirical Formula of an Oxide
Experimental Procedure
Prepare
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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 0.001 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
Exp 7: The Empirical Formula of an Oxide
Experimental Procedure
Heating
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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
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Repeat approximately every 3-5 min until no metal is visible
anymore
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Don’t open too far, because Mg will catch fire
Metal should glow bright-red
all is converted to magnesium oxide powder
no glowing is visible anymore
Allow crucible to cool
Exp 7: The Empirical Formula of an Oxide
Experimental Procedure
Analysis
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When the crucible has cooled down to the point where it is close
to room temperature
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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
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2 consecutive weightings within 0.001 g of each other
Exp 7: The Empirical Formula of an Oxide
Chemicals
Post-Lab Assignment due next week
• Fill in the Report Sheet (p. 115)
– Omit 9, 10, 11
– Show all your calculations on a separate sheet of paper.
• Answer questions 1 through 4 on Laboratory questions
following your calculations on the sheet of paper
• Critical Thinking (no need to write):
Think carefully about the precision of the masses that you
determined on the electronic balance. How many significant
figures are justified in your answer?
Also due: Pre-lab for next lab