Co^2+ + EDTA CoY^2- at pH = 6.0, = 2.3 times 10^-5, k_f = 10^16.31 = 2.04 times

Question

Co^2+ + EDTA CoY^2- at pH = 6.0, = 2.3 times 10^-5, k_f = 10^16.31 = 2.04 times 10^16, Therefore K_f = alpha_y4- K_f = 2.3 times 10^-5 times 2.04 times 10^6 = 4.7 times 1 V_e = 25.00 mL times 0.02026 mmol/mL/0.03855 mmol/mL = 13.14 mL 12.00 mL, there is excess Co^2+ in the solution. [Co^2+] = 25.00 mL times 0.02026 mmol/ML - 12.00 mL times 0.03855 mmol/mL/(25.00 + 12.00) mL = 1.19 times 10^-3 mmol/mL pCo^2- -log(1.19 times 10^-2) = 2.9 13.14 mL, [CoY^2-] = 25.00 mL times 0.02026 mmol/mL/(25.00 + 13.14) mL = 1.33 times 10^-2 mmol/mL Co^2+ + EDTA CoY^2- K'_f = [CoY2-]/Co2+][EDTA] = 1.33 times 10^-2 -x/x^2 = 4.7 times 10^11 x = 1.68 times 10^-7 mmol/mL 14.00 mL, [CoY^2-] = 25.00 mL times 0.02023 mmol/mL/(25.00 + 14.00) mL = 1.30 times 10^-2 mmol/mL [EDTA] = (14.00 - 13.14) mL times 0.03855 mmol/mL/(14.00+25.00) = 8.50 times 10^-4 mmol/mL [Co^2+] = [CoY^2-]/[EDTA]K'_f = 1.30 times 10^-2/8.50 times 10^-4 times 4.7 times 10^11 = 3.3 times 10^-11 mmol/mL pCo^2+ = -log(3.3 times 10^-11) = 10.49 Cu^2+ + Y^4- CuY^2- K_f = 10^18.80 = 6.3 times 10^13 alpha_y+ = 0.85 at pH 11.00 For Cu^2+ and NH_3, log beta_1 = 3.99, log beta_2 = 7.33, log beta_3 = 10.06, log beta_4 = 12.03. therefore, beta_1 = 9.8 times 10^3, beta_2 = 2.1 times 10^7, beta_3 -= 1.15 times 10^10, beta_4 = 1.07 times 10^12

Explanation / Answer

(b) Equivalence point

[CoY2-] formed = 1.33 x 10^-2 M

                            Co2+ + EDTA <==> CoY2-

initial                      -             -            1.33 x 10^-2

change                 +x          +x                 -x  

Eq                         x            x          (1.33x10^-2 - x)

with x amount of complex dissolved in solution at equilibrium, use data from ICE table

Kf' = 2.3 x 10^-5 x 2.04 x 10^16 = [CoY2-]/[Co2+][EDTA]

4.7 x 10^11 = (1.33 x 10^-2 - x)/(x)(x)

4.7 x 10^11x^2 = 1.33 x 10^-2 - x

4.7 x 10^11x^2 + x - 1.33 x 10^-2 = 0

In the form : ax^2 + bx + c = 0

solving the quadratic equation,

x = [-b (+/-) sq.rt.(b^2 - 4ac)]/2a

x = 1.68 x 10^-7 M

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