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

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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

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Summary: Mass, Moles, Numbers