大学化学（梁永晔）03-lecture-preview

5/6/2026Chapter3

Stoichiometry:

CalculationswithChemicalFormulasandEquationsJohnD.BookstaverSt.CharlesCommunityCollegeCottleville,MOLecturePresentationLawofConservationofMass“Wemaylayitdownasanincontestableaxiomthat,inalltheoperationsofartandnature,nothingiscreated;anequalamountofmatterexistsbothbeforeandaftertheexperiment.Uponthisprinciple,thewholeartofperformingchemicalexperimentsdepends.”--AntoineLavoisier,1789©2012PearsonEducation,Inc.ChemicalEquations

Chemicalequationsareconciserepresentationsofchemicalreactions.©2012PearsonEducation,Inc.AnatomyofaChemicalEquationCH4(g)+2O2(g)

CO2(g)+2H2O(g)©2012PearsonEducation,Inc.AnatomyofaChemicalEquationCH4(g)+2O2(g)

CO2(g)+2H2O(g)

Reactantsappearontheleftsideoftheequation.©2012PearsonEducation,Inc.AnatomyofaChemicalEquationCH4(g)+2O2(g)

CO2(g)+2H2O(g)

Productsappearontherightsideoftheequation.©2012PearsonEducation,Inc.AnatomyofaChemicalEquationCH4(g)+2O2(g)

CO2(g)+2H2O(g) Thestatesofthereactantsandproductsarewritteninparenthesestotherightofeachcompound.©2012PearsonEducation,Inc.AnatomyofaChemicalEquationCH4(g)+2O2(g)

CO2(g)+2H2O(g)

Coefficientsareinsertedtobalancetheequation.©2012PearsonEducation,Inc.SubscriptsandCoefficientsGiveDifferentInformationSubscriptstellthenumberofatomsofeachelementinamolecule.Coefficientstellthenumberofmolecules.©2012PearsonEducation,Inc.ReactionTypes©2012PearsonEducation,Inc.CombinationReactionsExamples:2Mg(s)+O2(g)2MgO(s)N2(g)+3H2(g)

2NH3(g)C3H6(g)+Br2(l)C3H6Br2(l)Incombinationreactionstwoormoresubstancesreacttoformoneproduct.©2012PearsonEducation,Inc.Inadecompositionreactiononesubstancebreaksdownintotwoormoresubstances.DecompositionReactionsExamples:CaCO3(s)

CaO(s)+CO2(g)2KClO3(s)2KCl(s)+O2(g)2NaN3(s)2Na(s)+3N2(g)©2012PearsonEducation,Inc.CombustionReactionsExamples:CH4(g)+2O2(g)

CO2(g)+2H2O(g)C3H8(g)+5O2(g)3CO2(g)+4H2O(g)

Combustionreactionsaregenerallyrapidreactionsthatproduce

aflame.Combustionreactionsmostofteninvolvehydrocarbonsreactingwithoxygenintheair.©2012PearsonEducation,Inc.FormulaWeights©2012PearsonEducation,Inc.FormulaWeight(FW)Aformulaweightisthesumoftheatomicweightsfortheatomsinachemicalformula.So,theformulaweightofcalciumchloride,CaCl2,wouldbeCa:1(40.08amu) +Cl:2(35.453amu)

110.99amu Formulaweightsaregenerallyreportedforioniccompounds.

©2012PearsonEducation,Inc.MolecularWeight(MW)Amolecularweightisthesumoftheatomicweightsoftheatomsinamolecule.Forthemoleculeethane,C2H6,themolecularweightwouldbeC:2(12.011amu)30.070amu+H:6(1.00794amu)©2012PearsonEducation,Inc.PercentComposition Onecanfindthepercentageofthemassofacompoundthatcomesfromeachoftheelementsinthecompoundbyusingthisequation:%Element=(numberofatoms)(atomicweight)(FWofthecompound)x100©2012PearsonEducation,Inc.PercentComposition Sothepercentageofcarboninethaneis%C=(2)(12.011amu)(30.070amu)24.022amu30.070amu=x100=79.887%©2012PearsonEducation,Inc.Moles©2012PearsonEducation,Inc.Avogadro’sNumber6.02x10231moleof12Chasamassof12.000g.©2012PearsonEducation,Inc.MolarMassBydefinition,amolarmassisthemassof1molofasubstance(i.e.,g/mol).Themolarmassofanelementisthemassnumberfortheelementthatwefindontheperiodictable.Theformulaweight(inamu’s)willbethesamenumberasthemolarmass(ing/mol).©2012PearsonEducation,Inc.UsingMoles Molesprovideabridgefromthemolecularscaletothereal-worldscale.©2012PearsonEducation,Inc.MoleRelationshipsOnemoleofatoms,ions,ormoleculescontainsAvogadro’snumberofthoseparticles.OnemoleofmoleculesorformulaunitscontainsAvogadro’snumbertimesthenumberofatomsorionsofeachelementinthecompound.©2012PearsonEducation,Inc.FindingEmpiricalFormulas©2012PearsonEducation,Inc.CalculatingEmpiricalFormulas Onecancalculatetheempiricalformulafromthepercentcomposition.©2012PearsonEducation,Inc.CalculatingEmpiricalFormulasThecompoundpara-aminobenzoicacid(youmayhaveseenitlistedasPABAonyourbottleofsunscreen)iscomposedofcarbon(61.31%),hydrogen(5.14%),nitrogen(10.21%),andoxygen(23.33%).FindtheempiricalformulaofPABA.©2012PearsonEducation,Inc.CalculatingEmpiricalFormulasAssuming100.00gofpara-aminobenzoicacid, C: 61.31gx =5.105molC H: 5.14gx =5.09molH N: 10.21gx =0.7288molN O: 23.33gx =1.456molO1mol12.01g1mol14.01g1mol1.01g1mol16.00g©2012PearsonEducation,Inc.CalculatingEmpiricalFormulasCalculatethemoleratiobydividingbythesmallestnumberofmoles:C: =7.0057H: =6.9847N: =1.000O: =2.00125.105mol0.7288mol5.09mol0.7288mol0.7288mol0.7288mol1.458mol0.7288mol©2012PearsonEducation,Inc.CalculatingEmpiricalFormulasThesearethesubscriptsfortheempiricalformula: C7H7NO2

©2012PearsonEducation,Inc.CombustionAnalysisCompoundscontainingC,H,andOareroutinelyanalyzedthroughcombustioninachamberliketheoneshownin

Figure3.14.CisdeterminedfromthemassofCO2produced.HisdeterminedfromthemassofH2Oproduced.OisdeterminedbydifferenceaftertheCandHhavebeendetermined.©2012PearsonEducation,Inc.StoichiometricCalculations Thecoefficientsinthebalancedequationgivetheratioofmolesofreactantsandproducts.©2012PearsonEducation,Inc.StoichiometricCalculations StartingwiththemassofSubstanceA,youcanuse

theratioofthecoefficientsofAandBtocalculatethemassofSubstanceBformed(ifit’saproduct)orused(ifit’sareactant).©2012PearsonEducation,Inc.StoichiometricCalculations Startingwith1.00gofC6H12O6… wecalculatethemolesofC6H12O6… usethecoefficientstofindthemolesofH2O… andthenturnthemolesofwatertograms.C6H12O6+6O2

6CO2+6H2O©2012PearsonEducation,Inc.LimitingReactants©2012PearsonEducation,Inc.LimitingReactantsThelimitingreactantisthereactantpresentinthesmalleststoichiometricamount.Inotherwords,it’sthere