4. - Determine the relation [NHA\'1/[NH OH] in orfer to obtain a pH o f 7.8 for

Question

4. - Determine the relation [NHA'1/[NH OH] in orfer to obtain a pH o f 7.8 for the buffer of pKa 9.25 Previously calculate the pOH S.- The following curve corresponds to the titration of maleic acid ( diprotic acid) with sodium hyddroxide 14 12 10 8 6 Figure 15-3 Titration curve for 25.00 mL of O.1000 M maleic acid, H2M, with 0.1000 M NaOH 4 2 o 10 20 30 40 50 60 Volume of O.1000 M NaOH, mL ) Chapter 15 p414 © 2004 Thomson Brooksoole a.- Determine the 2 values of the pKa b.- Determine the pH for both neutralizations points e volume of NaOH required to neutralized the first hydrogen and the second hydrogen

Explanation / Answer

5. From the curve for weak acid vs strong base titration

a. volume to reach first 1/2 equivalence point = 12.5 ml NaOH

pH at this point = 2.0

pKa1 = 2.0

volume to reach second 1/2 equivalence point = 37.5 ml NaOH

pH at this point = 6.3

pKa2 = 6.3

b. pH at fist equivalence point

let H2A be maleic acid

[HA-] = 0.1 M x 25 ml/50 ml = 0.05 M

HA- + H2O <==> H2A + OH-

let x amount hydrolyzes

Kb2 = [H2A][OH-]/[HA-]

1 x 10^-14/1 x 10-2 = x^2/0.05

x = [OH-] = 2.24 x 10^-7 M

pOH = -log[OH-] = 6.65

pH = 14 - pOH = 7.35

At second equivalence point

[A^2-] = 0.1 M x 25 ml/75 ml = 0.033 M

A^2- + H2O <==> HA- + OH-

let x amount hydrolyzed

Kb1 = [HA-][OH-]/[A^2-]

1 x 10^-14/5 x 10^-7 = x^2/0.033

x = [OH-] = 2.57 x 10^-5 M

pOH = -log[OH-] = 4.60

pH = 14 - pOH = 9.40

c) Volume NaOH to neutralize first hydrogen = 25 ml

Volume NaOH to neutralize second hydrogen = 50 ml

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