3. Titration pKa2 = 14 - pKb1 = 4.27 pKa1 = 14 - pKb2 = 1.25 a. Initial [C2O4^2-
ID: 700132 • Letter: 3
Question
3. Titration
pKa2 = 14 - pKb1 = 4.27
pKa1 = 14 - pKb2 = 1.25
a. Initial
[C2O4^2-] = 0.0655 M
C2O4^2- + H+ <==> HC2O4- + OH-
let x amount reacted
Kb1 = [HC2O4-][OH-]/[C2O4^2-]
1.84 x 10^-10 = x^2/0.0655
x = [OH-] = 3.48 x 10^-6 M
pOH = -log[OH-] = 5.46
pH = 14 - pOH = 8.54
b. after 2.00 HCl added
initial C2O4^2- = 0.0655 M x 16 ml = 1.048 mmol
added HCl = 0.17 M x 2 ml = 0.34 mmol
formed HC2O4- = 0.34 mmol
remaining C2O4^2- = 0.708 mmol
pH = pKa2 + log(C2O4^2-/HC2O4-)
= 4.27 + log(0.708/0.34) = 4.60
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after 8.00 HCl added
initial C2O4^2- = 0.0655 M x 16 ml = 1.048 mmol
added HCl = 0.17 M x 8 ml = 1.36 mmol
formed H2C2O4 = 0.312 mmol
remaining HC2O4- = 0.736 mmol
pH = pKa1 + log(C2O4^2-/HC2O4-)
= 1.25 + log(0.736/0.312) = 1.63
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after 20.00 HCl added
initial C2O4^2- = 0.0655 M x 16 ml = 1.048 mmol
added HCl = 0.17 M x 20 ml = 3.4 mmol
This is past second equivalence point
excess [HCl] = [H+] = 1.304 mmol/36 ml = 0.0362 M
pH = -log[H+] = -log(0.0362) = 1.44
c. pH at first equivalence point
pH = 1/2(pKa1 + pKa2) = 1/2(1.25 + 4.27) = 2.76
pH at second equivalence point
volume HCl added = 2 x 0.0655 M x 16 ml/0.17 M = 12.33 ml
[H2C2O4] formed = 1.048 mmol/28.33 ml = 0.037 M
H2C2O4 + H2O <==> HC2O4- + H3O+
let x amount reacted
Ka1 = [HC2O4-][H3O+]/[H2C2O4]
0.0562 = x^2/0.037
x = [H3O+] = 0.0456 M
pH = -log[H3O+] = 1.341
d. plot, volume of HCl (ml) on x-axis and pH on y-axis (file not uploading)
e. Indicator for this titration would be thymol blue for first equivalence and methyl orange for the second equivalence point detection. Best indicator should have color change in the region of equivalence point.
Explanation / Answer
3. A 16.00 mL aqueous solution of a 0.0655 M, sodium oxalate (Na2C204, pKbi -9.734, pKb2 12.750) is titrated with 0.170 M HC1. a. b. c. d. e. What is the initial pH of this solution? Calculate the pH of the solution after 2.00, 8.00, 20.00 mL of HCI is added. Calculate the pH at the equivalence point(s). Sketch the titration curve for this titration. Select appropriate indicator(s) for this titration.
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