Polvprotic Acids 2. Let Na:B be a tribasic salt. Consider the titration of 15.0

Question

Polvprotic Acids 2. Let Na:B be a tribasic salt. Consider the titration of 15.0 mL of 0.0250 M Na:B with 0.0350 M HCl. For H3B, Kal ta 1.2 x 103, Ka,-8.3 x 10", -3.6 x 10-12 a) What volumes of HCl is needed to reach the 1st, 2nd and 3rd equivalence points. b) At what pH range is HyB the dominate form present in solution? c) What is the pH at midpoint 1 and which conjugate pair is equal in concentration at midpoint 1? d) What is the pH at midpoint 2 and which conjugate pair is equal in concentration at midpoint 2? e) What is the pH at midpoint 3 and which conjugate pair is equal in concentration at midpoint 3? f) Which conjugate pair are present at pH 10? Circle the conjugate that is in higher concentration at pH 10? g) Calculate Kb, Kb2 and Kb, for the B3

Explanation / Answer

Solution:

Titration Na3B with HCl

initial moles Na3B = 0.0250 M x 15 ml = 0.375 mmol

a) volume of HCl to reach Ist equivalence point = 1 x 0.375 mmol/0.0350 M = 10.71 ml

volume of HCl to reach 2nd equivalence point = 2 x 0.375 mmol/0.0350 M = 21.43 ml

volume of HCl to reach 3rd equivalence point = 3 x 0.375 mmol/0.0350 M = 32.14 ml

b) pH range at which H3B would be present = -log(1.2 x 10^-3) = 2.92

so below pH 2.92 H3B only would be present in solution

c) pH at mid point 1 at which conjgate pair is equal in concentration = -log(3.6 x 10^-12) = 11.44

d) pH at mid point 2 at which conjgate pair is equal in concentration = -log(8.3 x 10^-7) = 6.08

e pH at mid point 3 at which conjgate pair is equal in concentration = -log(1.2 x 10^-3) = 2.92

f) conjugate pair present at pH 10 = HB-/B^3-

B^3- would be present in major amount.

g) Kb1 = 1 x 10^-14/3.6 x 10^-11 = 2.8 x 10^-3

Kb2 = 1 x 10^-14/8.3 x 10^-7 = 1.20 x 10^-8

Kb3 = 1 x 10^-14/1.2 x 10^-3 = 8.3 x 10^-12

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