Question

10.3 Titration curves are shown for two diprotic bases titrated with hydrochloric acid. Both bases are in the table of Ko values that is provided. Identify which base is titrated in each titration curve. Briefly explain how you know which base it is. (Hints: What is the relationship between Ko and PK, for a diprotic base? What is the relationship between pK, and pH in titration curves for weak acids and bases?) (a) Which weak base from the table is being titrated in this titration curve? ML HCl added (b) Which weak base from the table is being titrated in this titration curve? ml HCl added Diprotic base Kol Kb2 2-aminophenolate ion 9.3x10 5.0x100 carbonate ion 8.1 x 10" 1.3 x 10 ethanediamine 8.3x10 1.3x10 mercaptoacetate ion 1.3x10+ 3.0x10" 1,5-pentanedioate ion 1.1x101.5x10 piperazine 5.5x105 4.4x10°

Answer 1

For titration of dibasic weak base with strong acid,

two reactions occur :

B^2- + HCl -----> BH^- + Cl^-

BH^- + HCl -----> BH₂ + Cl^-

pOH = pK_b1 at half equivalence point of first reaction.

pOH = pK_b2 at half equivalence point of second reaction.

Titration curve 'a'

First half equivalence = 9.7

pOH = 14 - 9.7 = 4.3

pK_b1 = 4.3

K_b1 = 10^-4.3 = 5.01 x 10^-5

Second equivalence point = 5.5

pOH = 14 - 5.6 = 8.4

pK_b2 = 10^-8.4 = 3.98 x 10^-9

As the values match for piperazine,

hence, curve 'a' is for piperazine.

Titration curve 'b'

First half equivalence = 10

pOH = 14 - 9.9 = 4.1

pK_b1 = 4.1

K_b1 = 10^-4.1 = 7.94 x 10^-5

Second equivalence point = 7.1

pOH = 14 - 7.1 = 6.9

pK_b2 = 10^-7.0 = 1.26 x 10^-7

As the values match for ethanediamine,

hence, curve 'b' is for ethanediamine.