3 stoichiomitry Alevel 考试化学课件.pptx

Chapterthree

Atoms,moleculesandstoichiometryCalculationofempiricalandmolecularformulae

Empiricalformula:thelowestwholenumberratioofelementsinacompound.Molecularformula:theactualratioofelementsinacompound.Thetwocanbethesame.CH2C2H4C3H6H2Oempiricalformulamolecularformulabothmolecularformula

exampleIfmagnesiumisburnedinoxygen,magnesiumoxideisformed.Supposethatapieceofmagnesiumofknownmassisburnedcompletelyandthemagnesiumoxideproducedisweighed.Theweighingsenableustocalculatetheempiricalformulaofmagnesiumoxide:0.240gofmagnesiumribbonproduced0.400gofmagnesiumoxide:

MgOMass(g)0.2400.400-0.240=0.160=0.0100=0.0100Amount(mol)Magnesiumandoxygenatomsarepresentintheratio1:1.HencetheempiricalformulaofmagnesiumoxideisMgO

exampleAcompoundis43.64%Pand56.36%O.Whatistheempiricalformula?Theratiois

Combustionanalysis

CombustionanalysisIncombustionanalysis,allthecarbonisconvertedtocarbondioxideandallthehydrogentowater.Thecarbondioxideandwaterproducedarecarefullycollectedbyabsorptionandweighed.

calculatethemolecularformulaThecompositionbymassoforganiccompoundscanbefoundbycombustionanalysis.thisinvolvesthecompletecombustioninoxygenofasampleofknownmass.Incombustionanalysis,allthecarbonisconvertedtocarbondioxideandallthehydrogentowater.Thecarbondioxideandwaterproducedarecarefullycollectedbyabsorptionandweighted.calculationthengivesthemassofcarbonandhydrogenpresent.Iftheoxygenisalsopresent,it’smassisfoundbysubtraction.Otherelementsrequirefurtheranalyticaldeterminations.

CombustionAnalysisSummaryFindthemassofCandHinthesample.FindtheactualnumberofmolesofCandHinthesample.FindtherelativenumberofmolesofCandHinwholenumbers.Writetheempiricalformulafortheunknowncompound.

CombustionAnalysisNOTE!Instep1always