Determination of the Empirical Formula of Magnesium Oxide
Experiment 2
Experiment 2
Goal: To study stoichiometric relationships governing
mass and amount in chemical formulas.
Method: Measure masses before and after the oxidation
reaction of Mg metal and O2 gas
Use masses to calculate the experimental empirical formula of magnesium oxide
Compare the experimental empirical formula to the theoretical empirical formula
Practical ChemistryAtoms / molecules: hard to count
Formulas / equations: written based on numerical
relationships (not masses)
By using mass relationship to moles: Make something “unmeasurable”
measurable. Relate numbers of atoms and molecules to
“real world” values (masses).
Overview of Concepts
Law of conservation of mass Total mass of products must equal total mass of reactants
Law of constant composition Any portion of compound has same ratio of masses as
elements in compound (constant mass ratio)
Molecular composition expressed 3 ways Mass of each element per mole of compound Mass of each element present relative to total mass (mass %) # each type of atom of per molecule/formula unit (formula)
Empirical and Molecular Formulas
Empirical Formula “Simplest” formula Agrees with elemental analysis Smallest set of whole # ratio of atoms
Molecular Formula May be same as empirical “Na2Cl2” = NaCl
May be multiple of empirical H2O2 ≠ HO
CH2O
NameActual
FormulaMultiple
Molar mass
Use or function
Formaldehyde CH2O 1 30Disinfectant, bio
preservative
Acetic Acid C2H4O2 2 60 Polymers, vinegar
Lactic Acid C3H6O3 3 90Sours milk, product
of exercise
Erythrose C3H6O3 4 120Forms during sugar
metabolism
Ribose C5H10O5 5 150In many nuclei acids & vit B12
Glucose C6H12O6 6 180 Nutrient for energy
© McGraw-Hill Companies, Inc. Permission required for reproduction or display
Reaction SummaryHeating Mg metal in air:
1) Mg(s) + N2(g) + O2(g) MgO(s) + Mg3N2(s)
Converting Mg3N2 (add H2O):
2) MgO(s)+Mg3N2(s)+H2O(l) MgO(s)+Mg(OH)2(s)+ NH3(g)
Heating (conversion to magnesium oxide):
3) MgO(s) + Mg(OH)2(s) MgxOy(s) + H2O(g)
Crucible Set-up
Assume everything is hot once flames are on
Procedure
Heat empty crucible + lid Mass cool crucible + lid mcr+lid
Obtain ~0.3 g Mg ribbon Mass crucible + lid + Mg mcr+lid+Mg
Place lid with ~0.5cm gap
Heat gently ~1 min; strongly ~10 min (gray powder)
Cool; add ~ 1mL distilled H2O
Heat gently ~2-3 min; strongly ~5 min (gray powder) Mass cool crucible + lid mcr+lid+product
Mass balance and moles
Final product: MgxOy
Mass Mg:
Mass O:
Empirical formula:
lowest whole-# ratio Mg:O
MgxOy
lidcrproductlidcrOMg mmmyx
MgOMgo mmmyx
Mg
MgMg MM
massmol
O
OO MM
massmol
lidcrMglidcrMg mmm
Example Data
Using 0.3000 g Mg
If product is:Theoretical
yield (g)MgO 0.4974
Mg(OH)2 0.7198
Mg3N2 0.4153
Element or Molar Mass %MgCompound (g/ mol) (m/ m)
Mg 24.31 ---O 16.00 ---N 14.01 ---H 1.008 ---
MgO 40.31 60.31 "Mg-ideal"Mg(OH)2 58.33 41.68 "Mg-poor"
Mg3N2 100.95 72.24 "Mg-rich"
Theoretical ExperimentalMgused (g) 0.3000 Mgused (g) 0.3000
Mgused (mol) 0.0123 Mgused (mol) 0.0123
MgOexpected (mol) 0.0123 "MgO"obtained (g) 0.4652
MgOexpected (g) 0.4974 "MgO"obtained (mol) 0.0077 Mg0.0123 O0.0103
Oincorporated (g) 0.1974 Mg0.0123 O0.0123 Oincorporated (g) 0.1652 = Mg1.195 O1.000
Oincorporated (mol) 0.0123 = "MgO" Oincorporated (mol) 0.0103 = "Mg6 O5"%yield "MgO" 93.5 Mg-rich product
Share your results
Report your empirical formula on the board
Record class formulas
Discuss sources of error (include in discussion) Does your formula match expected one? MgO Does class data generally show expected results? Effects of:
Incomplete conversion to MgO Mg, Mg3N2, Mg(OH)2, …
Product losses – likely source of error?
Residual water – likely source of error?
Brainstorm: Keep logical; discard unlikely
Data/ResultsResult summary / sample calculations:
1. mass of Mg metal used
2. theoretical yield of MgO: Mg(s) + ½O2(g) → MgO(s)
3. mass of MgxOy produced4. mass of O incorporated into product
5. moles of Mg and O in MgxOy
6. empirical formula of the oxide (individual; class)
7. percent by mass of Mg and O in MgxOy
8. percent yield of the reaction: Mg + ½ O2 MgO
Discussion
Compare experimental empirical formula to theoretical empirical formula
Primary sources of error/deviation
Effects of factors such as: incomplete conversion of Mg3N2 to MgO, or
residual Mg(OH)2 in the product?
Does this method a valid way to determine empirical formula of metal oxides?
Report
Abstract
Results
Sample calculations
Discussion
Review questions
Fe 0.03171mol Fe 55.85g
Fe mol 1Fe g771.1
O 0.04229mol O 16.00g
O mol 1O .6776g0
OFe 0.042290.03171
Example #1
1.3341.000
0.03171
0.04229
0.03171
0.03171OFeOFe
2.6682.000
1.33421.0002OFeOFe
4.0023.000
1.33431.0003OFeOFe
1. Calculate no. moles each
2. Insert moles as subscripts
3. Divide by smallest no. moles
4. Multiply by 2,3…until whole #s
Formula: Fe3O4
2.448 g of an iron oxide was analyzed and found to contain 1.771 g Fe and 0.6766 g O. Calculate the empirical formula.
Na 0.1221mol Na 23.1g
Na mol 1Na .82g2
Cl 0.1227mol Cl 35.45g
Cl mol 1Cl .35g4
0.48940.12270.1221 OClNa
4.0081.0051.000
0.1221
0.4894
0.1221
0.1227
0.1221
0.1221OClNaOClNa
411
4.0081.0051.000OClNaOClNa
NaClO Formula 4
O 0.4894mol O 16.00g
O mol 1O .83g7
1. Calculate no. moles each
2. Insert moles as subscripts
3. Divide by smallest no. moles
4. Multiply by until whole #s
Elemental analysis of a sample of an ionic compound gave the following results: 2.82g of Na, 4.35g of Cl, and 7.83g of O. What is the empirical formula?
Example #2
C 3.331mol C 12.01g
C mol 1C 0.0g4
H 6.657mol H 1.008g
H mol 1H .71g6
3.3316.6573.331 OClC
1.0001.9981.000
3.331
3.331
3.331
6.657
3.331
3.331OHCOHC
121
1.0001.9981.000OH COHC
:Empirical OCH2
O 3.331mol O 16.00g
O mol 1O 3.3g5
1. Calculate no. moles each
2. Insert moles as subscripts
3. Divide by smallest no. moles
4. Multiply by until whole #s
Lactic acid (90.08g/mol) forms in muscle tissue and causes muscle soreness. It is 40.0 mass% C, 6.71 mass% H, and 53.3 mass% O. Find the empirical formula.
Example #3
Empirical Formula
©Pearson Education, Inc., publishing as Benjamin-Cummings Permission required for reproduction or display
Summary: Mass, Moles, Numbers