Definitionofmole
Basicparticlesshouldbespecifiedwhenusingmol,whichcanbeatoms,molecules,ions,groupsofatoms,electrons,protons,neutronsandotherparticles,Oraspecificcombinationoftheseparticles.Accordingtointernationalregulations,thenumberofparticlescontainedin1molofparticlesis6.02×10²³,andthenumberofcarbonatomscontainedin0.012kg¹²C(carbon-12)isalsoequalto6.02×10²³.Therefore,onemoleofmattercontainsstructuralparticles.Thenumberofisequaltothenumberofatomscontainedin0.012kg¹²C(carbon12).
Sometimes,themassofonemoleofasubstanceiscalledthemolarmassofthesubstance,whichisrepresentedbythesymbolM,suchasM=2.02×10⁻³kgforhydrogenH2.ForasubstancewhosemassFisM,theratioofMtoμiscalledtheamountofthesubstance(alsoknownasthenumberofmoles),=Mμ.Forexample,M=4.04×10⁻³kgThenumberofmolesofhydrogenH₂=2.ThevolumeVmoccupiedbyonemoleofsubstanceiscalledthemolarvolume.Themolarvolumeofgasdependsontemperatureandpressure.Understandardconditions,theidealgasVm=22.41410L.Fmol⁻¹.Themolarvolumeofsolidandliquidsubstanceshasasmallrelationshipwithtemperatureandpressure.Thevolumeofamoleofdifferentsolidsubstancesanddifferentliquidsubstancesisdifferent.
Accordingtotheprecisemeasurementofscientificexperiments,itisknownthat0.012kgofacarbonatom¹²C(carbon-12)containsabout6.02×10²³.
Whyuse"12grams"and"carbon¹²C(carbon-12)"todefinemole?Themainreasonsareasfollows(forsecondaryschoollearningonly):
Whydoyouhaveto12grams?Becauseitsnucleuscontainsequalnumbersofneutronsandprotons.Whycanthenumberofneutronsandprotonsbeequaltoeachotherasthereferenceatomforthenumberofmoles?Becauseaneutronhasonemorenegativeelectronandsomeradiationthanaproton,strictlyspeaking,asingleneutronhasaslightlyhighermassthanaproton[itsmassis1.6749286×10⁻²⁷kg(939.56563MeV),whichisslightlyhigherthanthemassofaproton.Large(themassoftheprotonis1.672621637(83)×10⁻²⁷kg)scientificcountingistoocomplicated],inordertofacilitatethemeasurementofthemassofothernuclei,itisnecessarytotakeanintermediatevalueoftheneutronmassandtheprotonmass(theaveragevalueistraditionalMeasurementmethods).Then¹²C(carbon-12)nucleusisthebestchoiceasareferencebenchmark,anditexistsmorecommonlyanditspropertiesarerelativelystable.Inthisway,onetwelfthofthemassofthe¹²C(carbon12)nucleusisscientificallytheclosesttothemassofaneutronorproton.
Acertainnumber(Avogadro'sconstant,oronemole)ofneutrons(protons-hereyoucanregardneutronsandprotonsasparticleswiththesamemass)isbasicallyequalto1gram,thenThenucleusofonemoleofXelement[containingm(naturalnumber)protonsandn(naturalnumber)neutrons]ism+ngrams.Andm+nisexactlytheatomicweight.mdeterminesthepositionoftheatomintheperiodictableofelements,andndeterminestheatomicweightofatomsofthesameelement.
OnNovember16,2018,theInternationalConferenceonWeightsandMeasurespassedaresolutionthat1molewillbedefinedas"theamountofmatterinasystemcontainingexactly6.02214076×10²³basicunitssuchasatomsormolecules".Atthesametime,theAvogadroconstantwasrevisedto6.02214076×10²³.
Basicinformation
Scientifically,theaggregatecontaining6.02×10²³particlesisregardedasaunit,calledmole,whichisaunitthatrepresentstheamountofmatter(symbolisn).Abbreviatedasmol,theunitsymbolismol.
1molofcarbonelementcontains6.02×10²³carbonatoms,andthemassis12grams.
1molofelementalsulfurcontains6.02×10²³sulfuratomsandthemassis32grams.
Similarly,themassof1moleofanysubstanceisingrams,andthevalueisequaltotherelativeatomicmassorrelativemolecularmassofthatkindofatom(afixedvalue).
Theformulaweight(relativemolecularmass)ofwateris18,andthemassof1molwatermoleculeis18g,containing6.02×10²³watermolecules.
Usuallythemassof1molsubstanceiscalledthemolarmassofthesubstance(symbolisM),theunitofmolarmassisgrams/mol,pronouncedas"grampermole"(symbolis"g/mol")Forexample,themolarmassofwateris18g/mol,writtenasM(H₂O)=18g/mol.
Whatistherelationshipbetweenthemassofasubstance(m),theamountofasubstance(n),andthemolarmass(M)ofasubstance?
Thatis:n=m/M,m=n×M,M=m/n
Generalformula:n(amountofmatter)=N(numberofparticles)/NA(Avogadroconstant)=m(mass)/M(molarmass)=V(gasvolume)/Vm(molarvolume:gasinSTP(standardcondition:273K(0℃)101KPa)1molgasvolumeis22.4L)=C(substanceconcentration)×V(totalvolumeofsolution)
Writingmethod:coefficient+mol+chemicalformula(orChinesecharacters,note:Chinesecharacters)Mustindicatewhetheritisamoleculeoranatom,etc.)
Forexample:onemoleofwatermoleculecanbewrittenas1molH₂Oor1molwatermolecule
Developmenthistory
InOctober1971,the14thInternationalConferenceonWeightsandMeasures,attendedby41countries,decidedtoaddtheseventhbasicunitoftheInternationalSystemofUnits(SI).Molarisusedtocalculatethenumberofparticles,themassofmatter,thevolumeofgas,theconcentrationofthesolution,theheatchangeofthereactionprocess,andsoon.MolecomesfromtheLatinmoles,whichoriginallymeansalotandaccumulation.
Thedefinitionofmoleinthe14thInternationalConferenceonWeightsandMeasuresin1971hasthefollowingtwoparagraphs:"Moleistheamountofsubstanceinasystem,andthenumberofbasicunitscontainedinthesystemisequalto0.012kgofcarbon-Thenumberofatomsof12isequal.""Whenusingmoles,thebasicunitshouldbespecified.Itcanbeatoms,molecules,ions,electronsandotherparticles,oraspecificcombinationoftheseparticles."Thetwoparagraphsaboveshouldberegardedasoneoverall.Thenumberofcarbonatomscontainedin0.012kgofcarbon-12nuclideistheAvogadroconstant(NA),andtheapproximatevaluemeasuredbytheexperimentisNA=6.02×10²³.Moleisdifferentfromgeneralunits.Ithastwocharacteristics:①Theobjectofmeasurementismicroscopicbasicunits,suchasmolecules,ions,etc.,anditcannotbeusedtomeasuremacroscopicsubstances.②ItusesAvogadro'snumberastheunitofmeasurement,whichisabatch,notthenumberofmolecules,atomsandotherparticles.Itcanalsobeusedtomeasurespecificcombinationsofmicroscopicparticles.Forexample,theamountofsulfuricacidismeasuredinmoles,thatis,1molofsulfuricacidcontains6.02×10²³sulfuricacidmolecules.Moleisthemostwidelyusedunitofmeasurementinchemistry.Forexample,itisusedinthecalculationofchemicalreactionequations,calculationinsolution,solutionpreparationanddilution,calculationofchemicalequilibrium,gasmolarvolume,andthermochemistry.basicunit.
Molarextinctioncoefficient,Molarabsorptioncoefficient,Molarabsorptioncoefficient
Spectrophotometryisbasedontheselectiveabsorptionofelectromagneticradiationbysubstanceswithdifferentmolecularstructures.Theestablishedmethodbelongstomolecularabsorptionspectrumanalysis.Whenlightpassesthroughthesolution,themeasuredsubstancemoleculesabsorbacertainwavelengthofmonochromaticlight,andtheintensityoftheabsorbedlightisproportionaltothedistancethelightpasses.AlthoughitisunderstoodthatBouguerhadproposedthemathematicalexpressionoftheaboverelationshipasearlyas1729,itisgenerallybelievedthatLambertfirstdiscoveredtheexpressionin1760,anditsmathematicalformis:
T=I/I0=10(-kb)
whereI0istheincidentlightintensity,Iisthetransmittedlightintensity,10(-kb)istheindexbasedon10,kisaconstant,bistheopticalpathlength(usuallyexpressedincm).
Beer'slawisequivalenttoBouguer'slaw,butBeer'slawisexpressedintermsofconcentration.CombinethetwolawstoformtheBeer-Bouguerlaw:
T=I/I0=10(-kb)
wherecistheconcentrationofthelight-absorbingsubstance(usuallyexpresseding/Lormg/Lastheunit).Aftertakingthelogarithmwiththebase10astheaboveformula,alinearexpressionisobtained:
A=-logT=-log(I/I0)=log(I0/I)=εbc
whereAistheabsorbanceandεisthemolarabsorptioncoefficientorextinctioncoefficient.
TheaboveexpressionisusuallycalledBeer'slaw.Itshowsthatwhenmonochromaticlightofaspecificwavelengthpassesthroughthesolution,theabsorbanceofthesampleisproportionaltotheconcentrationoftheabsorberinthesolutionandthedistancethelightpasses.
Whenthewavelength,solutionandtemperaturearedetermined,themolarextinctioncoefficientisdeterminedbythecharacteristicsofagivensubstance.Infact,themeasuredmolarextinctioncoefficientisalsorelatedtotheinstrumentused.Therefore,inquantitativeanalysis,themolarextinctioncoefficientofaknownsubstanceisusuallynotused,butoneormoreknownconcentrationsofthesubstancetobetestedareusedasacalibrationorworkingcurve.
Absorptioncoefficient
ThemathematicalexpressionofBeer'slawisA=kbc,iftheconcentrationofthesolutioncising/L,bistheopticalpathincm,thentheconstantKiscalledtheabsorbancecoefficient,expressedbya,anditsunitisliter/(g·cm)[L/(g·cm],A=kbccanbewrittenasA=abc.
IntheformulaA=kbcIs1mol/L,bis1cm,thenthecoefficientkiscalledmolarabsorptioncoefficient,expressedbyε,theunitisliters/(mol·cm)[L/(mol·cm)],A=kbccanbewrittenasA=εc.Inactualwork,youcannotdirectlymeasuretheabsorbancewithahigh-concentrationsolutionsuchas1mol/L,butcalculatetheabsorbancewhendilutedtoanappropriateconcentration.Theεvalueisrelatedtofactorssuchasthewavelengthoftheincidentlightandthenatureofthesolution.TheεofNADHat260nmis15000,whichiswrittenasε260NADH=15×10³;theεat340nmis6220,whichiswrittenasε340NADH=6.22×10³.
Forexample,cintheformulaA=kbcisthepercentageconcentration(w/v)biscm,thentheconstantkcanbeexpressedbyE%,whichiscalledthespecificabsorptioncoefficientorthepercentageabsorptioncoefficient,A=kbccanbewrittenasA=E%bc.Whenthechemicalstructureoftheanalyteisknown,εcanbeusedForvalueanalysis,ifthechemicalstructureofthetestsubstanceisunknown,εcannotbedetermined.Atthistime,itisveryconvenienttousethespecificabsorptioncoefficienttoanalyze.a,εandEareoftenusedforroughquantitativeanalysis,mainlyforqualitativeanalysis.
Chemicalequation
Chemicalequationcanexpresstheratiooftheamountofthesubstancebetweenthereactantandtheproductandtheratioofthemass.
Forexample:2H₂+O₂=(ignite)=2H₂O
Theratioofthecoefficientsis2:1:2
Theratioofthenumberofparticles2:1:2
Theratiooftheamountofthesubstance2:1:2
Massratio4:32:36
Fromtheaboveanalysis,itcanbeseenthattheratioofthecoefficientsofeachsubstanceinthechemicalequationistheratiooftheamountofsubstancesbetweenthem.UsethisTheprinciplecanbebasedonthechemicalequationtocalculatetheamountofeachsubstance.
Onetwelfthofthemassofacarbonatom(¹²C)istheinternationalbenchmarkforrelativeatomicmass(formulaweight).Itisknownthat12g¹²Ccontains6.02214076×10²³carbonatoms.ThisnumberiscalledAvogadro'snumber,soitcanalsobesaidthattheamountofasubstancecontainingseveralbasicparticlesofAvogadrois1mol.Forexample,1molofoxygencontains6.02214076×10²³oxygenmolecules,itsmassis31.9988g.1molhydrogenioncontains6.02214076×10²³hydrogenions,anditsmassis1.00794g.Inthechemicalcalculation,theapproximatevalueis6.02×10²³.