2022_H1_Theories of acids and bases_assignment (teacher).doc

St. Andrew’sJunior CollegeH1Chemistry2022AssignmentTheoryof AcidsandBases 1. Norepinephrine is an organic chemical that functions in the brain and body as ahormoneandneutrotransmitter.Norepinephrinehasthestructural formulashownbelow: Norepinephrine,abbreviatedasBNH2 ,undergoespartial dissociationasfollows:BNH2 +H2 O BNH3 + +OH - (a) DefinepOH.pOH=-log10 [OH – ] (b) Calculate the [OH − ] that has dissociated from Norepinephrine given that thedegree of dissociation is 0.0450 for 0.05 mol dm -3 of Norepinephrine at 25 °C.Hence,calculatethepHofthesolution.[OH − ]/0.05=0.045[OH − ] =2.25 x10 -3 mol dm -3 .pOH=2.65pH=14–2.65=11.4 (c) WriteanexpressionforKb andcalculateitsvaluegiventhat[BNH3 + ]=[OH - ].Kb =[BNH3 + ][OH − ]/[BNH2 ]Kb =1.01x10 -4 mol dm −3 (d) When hydrochloric is reacted with excess Norepinephrine, a buffer solution isformed. Explain with the aid of two equations to explain how the solution cancontrol pH.OH – + BNH3 + BNH2 + H2 OThesmall amount of OH – ionsareremovedbythelargeamount of BNH3 + in 1

thebuffer.H + + BNH2 BNH3 + Thesmall amount of H + ionsremovedbythelargeamount of BNH2 inthebuffer.ThereforepHremainsalmostunchanged. (e) SuggestasuitableindicatorforthetitrationofNorepinephrineandhydrochloricacidandexplainthereason.Methyl orange.Titrationofweakbasewithstrongacidwill resultinacidicsaltformedandtheworkingrangeoftheindicatorlieswithintherapidpHchangeofthetitrationcurve. (f)* Explain with the aid of an equation how BNH3 + undergoes partial hydrolysis inwater.BNH3 + +H2 O BNH2 +H3 O + 2