Mastery Problem Set M6 - Practice questions PDF

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CHEM1011/1031) Mastery(Practice(Problems( Topic(6:(Acids(and(Bases( • • •

Straightforward,question,from,Mastery,lecture Single,concept Taught,in,lectures

Q1.!(Choosing!a!buffer!combination):!!!Consider!the!following!table!of!weak!acids!and! their!relevant!pKa.!!Place!a!“✓”!in!the!box!to!indicate!which!acid,!in!combination!with!its! conjugate!base,!would!make!the!best!buffer!solution.!!Indicate!with!a!“+”,!any!buffer! combination!that!you!think!would!be!appropriate,!but!not!the!best.! Target!pH! Acid, pKa, 7.0, 9.0, 5.0, H2CO3, 6.3, HCO3–, 10.3, – H2PO4 , 7.2, HPO42–, 12.4, CH3COOH, 4.7, NH4+, 9.3, HCN, 9.2, “Rule!of!Thumb”,,Buffer,solutions,can,be,made,up,readily,for,pH,within,1,unit,of,the,pK., Q2.!(Preparing!a!buffer):!!What!mass!of!NaOH!(s)!must!be!added!to!the!same!500!mL! solution!of!0.40!M!NaHCO3!(aq)!to!give!the!pH!9.50!buffer!solution,!given!pKa!(HCO3–)!!=!! 10.25?!!(Assume!no!change!in!volume!when!the!solid!is!added.)! Write,equation:,

Henderson-Hasselbalch:,

Q3:!!Which!of!the!following!species!would!you!classify!as!Lewis!acids!or!bases?! Species,

Lewis,acid,or,base?,

K+, Al3+, F– , H2O,

Q4:!!Are!the!following!solutions!acidic,!basic!or!neutral?! i)

0.01,M,,KNO3?

ii)

0.10,M,,NaCN

iii)

0.10,M,,NH4Cl

iv)

0.10,M,,CH3COONH4

Q5:!!Using!the!Henderson-Hasselbalch!equation,!what!is!the!pH!of!a!solution!consisting! of!0.1!M!NH3!and!0.05!M!!NH4Cl?!!!![Data:!!pKb!(NH3)!=!4.7]!

! • Requires,concepts,from,multiple,lessons! ! , ! Q6:!!Consider!the!following!titration!curve!for!the!titration!of!0.10!M!maleic!acid!(HOOCCH2=CH2-COOH,!see!structure!below)!with!0.10!M!NaOH.!!The!pH!of!the!equivalence! points!is!shown.!! i) Choose!an!indicator!from!those!in!the!lecture!notes!that!is!suitable!for!each! equivalence!point.!!! ii) Without!doing!calculations,!estimate!pKa(1)!and!pKa(2)!for!the!two! carboxylate!protons.! ! Maleic,acid

,

! , , ! Q7:!!Consider!the!following!equilibrium!acid-base!reaction,!which,!at!a!certain! temperature!has!a!pKa!=!6.0! , , H2CO3,(aq),+,H2O,(l),,⇌,,,HCO3–,(aq),+,H3O+,(aq);, , pKa,=,6.0, , What,is,the,value,of,K,for,the,following,reactions?, K! , ,

i),,HCO3–,(aq),+,H2O,(l),⇌,,,H2CO3,(aq),+,OH–,(aq),

,

,

ii),,HCO3–,(aq),+,H3O+,(aq),⇌,,,H2CO3,(aq),+,H2O,(l),

,

,

iii),H2CO3,(aq),+,OH–,(aq),⇌,,HCO3–,(aq),+,H2O,(l),

,

! ! , , !

!

Q8:!!(Difficult!question!)!!Consider!a!1.00!L!buffer!made!from!0.040!M!Na2HPO4!(aq)!and! 0.080!M!KH2PO4!(aq),!given!pKa!(H2PO4–)!=!7.21.!!What!is!the!pH!of!the!buffer!and!how! much!10.0!M!HCl!can!I!add!before!the!pH!moves!1!unit!to!lower!pH?!(Assume!initially!no! dilution!by!adding!strong!acid,!but!check!afterwards).! , Write,equation:, , , Henderson-Hasselbalch,to,get,pH:,,, , , , , , , , , , What,is,target,pH,after,adding,strong,acid?, , , , , , What,is,ratio,of,base,to,acid,here?, , , , , , , What,is,total,acid,at,this,pH?, , , , , , How,much,acid,have,I,added?, , , , , , , , , , , , , , , , ,

, , • • • • ,

Requires,interpretation,of,a,long,,written,question! Requires,explanation,of,scientific,ideas,in,words! Requires,knowledge,of,other,lessons! Requires,logical,extension,of,knowledge,from,lessons!

! Q9:!Chloroacetic!acid,!ClCH2COOH,!is!used!as!an!herbicide.!It,is,a,weak,acid,with,Ka,=, 1.4E-3.,What,is,the,pH,of,a,0.070,M,solution,of,chloroacetic,acid?,Show,and,explain,your, work., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,...