Exp 7: Empirical Formulas

1
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
• the identity of the atoms
• the relative number of each type of atom

2
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
• By pieces of matter atoms, molecules, ions
• 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

3
Exp 7 The Empirical Formula of an Oxide

• Mass percentage
• By definition masspart
• 
  masswhole
• Example 0.192 g P reacts with oxygen to form
  0.440 g of a phosphorus oxide. What is the
  composition?
• mass of oxygen in the oxide
• (total mass of the oxide) (mass of
  phosphor)
• (0.440 g 0.192 g) 0.248 g oxygen
• Mass O (0.248 g/0.440 g) x 100 56.4
• Mass P (0.192 g/0.440 g) x 100 43.6

Mass ___________ x 100
4
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
• 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

5
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

6
Exp 7 The Empirical Formula of an Oxide

• Mole percentage and empirical formula
• 0.192 g P reacts with oxygen to form 0.440 g of a
  phosporus oxide
• mass of oxygen in the oxide 0.248 g oxygen
• molar mass of P 30.98 g/mol
• molar mass of O 15.9994 g/mol
• of moles of O 0.248 g/15.9994 g/mol 0.0155
  mol 1.55 x 10-2 mol
• of moles of P 0.192 g/30.974 g/mol 0.0062
  mol 6.2 x 10-3 mol
• We have 1.55 x 10-2 mol O and 6.2 x 10-3 mol P
• Ratio O/P 1.55 x 10-2 mol/ 6.2 x 10-3 mol 2.5
• Empirical formula P1O2.5
• You cannot split atoms ? it needs to be a
  whole-number ratio
• ? P1O2.5 P2O5 (empirical formula)
• Molar percentage of P 2 mol/7 mol x 100 28.6
• Molar percentage of O 5 mol/7 mol x 100 71.4

7
Exp 7 The Empirical Formula of an Oxide

• Possible Reaction Equation
• Elemental phosphorous P4
• Elemental oxygen O2
• Reaction equation
• P4 O2 ? P2O5
• Needs balancing (Law of Mass Conservation)
• number of atoms in the reactants (left of the
  arrow) number of atoms in product(s) (right of
  the arrow)
• balanced equation
• P4 5 O2 ? 2P2O5

8
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

9
Exp 7 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

10
Exp 7 The Empirical Formula of an OxideChemicals

• Magnesium
• Formula Mg
• Mole mass 24.305
• Melting point 650 oC
• Boiling point 1107 oC
• Density 1.74 g/cm3
• Water Solubility Insoluble/reactive
• Appearance Silvery to grayish-white metal
• Comments Flammable solid
• Magnesium oxide
• Formula MgO
• Mole mass 40.3044
• Melting point 2800
• Boiling point 3600
• Density 3.58 g/cm3
• Water Solubility Insoluble
• Appearance White fume
• Comments Moisture-sensitive

• Oxygen
• Formula O2
• Mole mass 31.9988
• Melting point -218.4 oC
• Boiling point -183.17 oC
• Vapor Density 1.1053 g/cm3
• Water Solubility Slightly soluble
• 0.004 g/100 ml
• Appearance Colorless gas blue liquid
• Comments non-flammable gas

11
Exp 7 The Empirical Formula of an
OxideExperimental 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 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

12
Exp 7 The Empirical Formula of an
OxideExperimental 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
• Dont 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

13
Exp 7 The Empirical Formula of an
OxideExperimental Procedure

• Analysis
• When the crucible has cooled 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 0.001 g of each
  other

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Exp 7 The Empirical Formula of an OxideChemicals

• 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