A-LEVEL考试：原子分子的计算复习
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今天给同学们分享下刘欣梦老师给大家讲解的A-LEVEL考试中的原子分子的计算复习。
Part1TheMole
1Relativeatomicmass
Therelativeatomicmass,Ar,ofanelementistheaveragemassofthenaturallyoccurringisotopesoftheelementrelativetothemassofanatomofcarbon-12.
Therelativemolecularmass,Mr,isthemassofamoleculerelativetothemassofanatomofcarbon-12,whichisgiventheexactmassof12.
Example:NH3=14+3×1=17.
Therelativeformulamassisusedforcompoundsmadeupofions.
2ThemoleandAvagadrosnumber
Amoleofsubstanceistheamountofsubstancethathasthesamenumberofparticlesastherearein12.00gofcarbon-12.Theparticlesmaybeatoms,molecules,ionsorevenelectrons.
ThisnumberofparticlesisaconstantknownasAvagadrosconstant(L),andisapproximately6x1023mol-1.
Themassofonemoleofasubstanceisoftenreferredtoasthemolarmass(M).Theunitsofmolarmassaregmol-1.
Tofindtheamountofsubstancepresentinagivenmass,wemustdividethatmassbythemolarmassofthesubstance.
Forexample-ifwehad10gofCaCO3:
Mis40+12+3×16=100g.
Soin10gofcalciumcarbonatethereis10/100mol=0.1molCaCO3.
Tofindthemassofagivensubstance,wemultiplythenumberofmolesofthesubstancebythemolarmass.
3Calculationsofmassfromstoichiometricequations
Ifweknowthemassofareactant,wecanfindthemassofaproductinachemicalequation.
Considertheformationofammoniafromhydrogenandnitrogen.
2N2(56g)+3H2(6g)→4NH3(34g)
Hence,forevery6gofhydrogenwemake34gofammonia.
Part2MassSpectrometry
1DeterminationofArandMr
Aninstrumentcalledamassspectrometerisusedtocalculaterelativeatomicmass.
Inamassspectrometer,atomsareconvertedtopositiveions;thesearethendeflectedbyelectricandmagneticfieldsbeforebeingdetected.Themassoftheelementsindividualisotopesarefoundaswellastheirabundance.
Exampleofamassspectrumisshownbelow:
FromthisdatatheRelativeAtomicMasscanbefound.
Part3EmpiricalandMolecularFormulae
1Definitions区分经验式和分子式的关系
Theempiricalformulaofacompoundshowsthesimplestwhole-numberratiooftheelementspresent.
Themolecularformulashowsthetotalnumberofatomsofeachelementpresentinamoleculeofacompound.
Example:
hexane:C6H14
EmpiricalFormula:C3H7
MolecularFormula:C6H14
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