265 Assignment 10
[2]
1. What will be the conditional formation constants for the following reactions at
25oC in an aqueous solution? Assume that the only significant side reactions are
the acid-base reactions of EDTA.
(a) Cu2+ + EDTA at pH 4.0
Cu2+EDTA: log Kf = 18.78
Kf =
For EDTA, 4 = at pH 4 = 3.0 x 10-9
K’f = 2.4 x 10-9
(b) Ni2+ + EDTA at pH 9.0
Ni2+ EDTA: log Kf = 18.4
For EDTA, 4 = at pH 9 = 0.041
K’f = 2.3 x 1017
[4]
2. A 25.00 mL sample containing 0.01250 M Ca2+ is titrated at pH 10.0 using a
0.01050 M solution of EDTA as the titrant.
(a) Write the equation for the titration reaction for this analysis and determine the
volume of titrant that is required to reach the endpoint.
Ca2+ + Y4- ⇋(YCa)2Or
Ca2+ + EDTA4- ⇋ (EDTACa)2mmol Ca2+ = 25.00 mL x 0.0125 mmol/mL = 0.3125 mmol
0.3125 mmol/0.01050 mmol/mL = 29.76 mL
(b) Calculate the value of pCa for the sample/titrant mixture at the beginning of the
titration, at the equivalence point, and after 10.00 and 30.00 mL of titrant has been
added.
At 0 ml: pCa = 1.903
At equivalence point: K’CaY = 0.36 * 4.8978x1010 = 1.76 x 1010
[CaY] = 0.0003125/0.05476 L = 0.00570672 M
1.76 x 1010 = 0.00570672/[Ca2+]2 [Ca2+] = 5.7 x 10-7
pCa = 6.24
At 10 mL added :
mmol Ca2+ = 25.00 mL x 0.0125 mmol/mL = 0.3125 mmol
mmol EDTA = 10.00 mL *0.01050 mmol/mL = 0.1050 mmol
mmol Ca2+ remaining = 0.3125 mmol-0.1050 mmol = 0.2075 mmol
0.2075 mmol/35.00 mL = 0.005929 M pCa = 2.2270
At 30.00 mL added:
mmol Ca2+ = 25.00 mL x 0.0125 mmol/mL = 0.3125 mmol
mmol EDTA = 30.00 mL *0.01050 mmol/mL = 0.3150 mmol
‘All’ the Ca2+ will react to form CaY
XS EDTA = 0.0025 mmol
[𝐶𝑎𝑌]
K’CaY = [𝐶𝑎2+][𝑦 4−] =
[Ca2+] = 7.1 x10-9
pCa= 8.15
[2]
0.3125mmol/V
mmol
[0.0025
∗[Ca2+ ]
V
= 1.76 x 1010
3.Use the Table on page 251 of your text to determine if you can titrate the
following ions in the presence of Mg2+
Ba2+ pH >11.0, methyl thymol blue
Ni2+ pH 4, pyridylazonaphthol plus Cu-EDTA
Hg2+ pH < 9 , Cu-PAN or Chromozurol s
Cd2+ pH 5, pyridylazonaphthol plus Cu-EDTA or Xylenol Orange or
Methylthymol blue
For each, say what indicator you would use, what pH you would use, and if an auxiliary
complexing agent is necessary.
[2]
From the text
11.23
11-24
11-30
11-14