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CHAPTER3CALCULATIONSWITHCHEMICALFORMULASANDEQUATIONS
I.Molecularmass(weight)andFormulamass(weight)A)Molecularmassisthesumoftheaveragemassesofalltheatomsinamolecule.Thisgivesusanaveragemolecularmass.Why?Wewillgenerallyroundofftohundredths-exceptforhydrogenwewillusethe1.008amu.
Someexamples:Whatisthemolecularmassofwaterinamu?Whatdoyouhavetoknowtoanswerthequestion?2H+1O[2(1.008)+1(16.00)=18.02amu(u)]forglucose-C6H12O6-6(12.01)+12(1.008)+6(16.00)=180.16u
B)Formulamassisthesumoftheaveragemassesofalltheatomsinaformulaunitofanioniccompound.NaCl1(22.99)+1(35.45)=58.44uAl2(SO4)3Youmustbeabletocountthenumberofeachkindofatom(ion)sothatyouknowbywhatnumbertomultiplythemassonthechart.2(26.98)+3(32.07)+12(16.00)=342.17u
II.Howdowecountatoms,molecules,andions?A)ELEMENTMEASURINGAtomsandmoleculesareextremelysmall.Sincewecantweighindividualatoms,molecules,orformulaunits,wemustresorttoweighinglargenumbersofthese.
Scientistshavedevelopedaconcepttoallowforalogicalmethodofweighingalargenumberofatoms,molecules,andformulaunits.Thisconceptisknownas:THEMOLEOnemoleofasubstancerepresentsaLARGEnumberofatomsormoleculesorformulaunits.ThemoleisanSIUnitwhichrepresentsanamountofsubstance.
Itisawayoflinkingthemacroscopicworldwiththesub-microscopicworld.Itisreadilyseenthatequalnumbersofmolescontainequalnumbersoffundamentalparticles,betheyatoms,molecules,formulaunits,ions,electrons,protons,etc.Themoleallowsustouseratioswhencomparingamountsofdifferenttypesofatoms,molecules,orformulaunits.
B)THEMOLE(mol)isdefinedasthequantityofagivensubstancethatcontainsthesamenumberofunits(atoms,molecules,formulaunits)asthereareC-12atomsinexactly12gramsofpureC-12.C)Rece
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