462

Instrumental Analysis—Chem 462

Homework Set #4

1. Calculate the potential of a silver electrode in contact with the following:

a. a solution that is 0.0200 M in I - and saturated with AgI.

b. a solution that is 0.00600 M in CN - and 0.0400 M in Ag(CN)

- .

c. the solution that results from mixing 25.0 mL of 0.0400 M KBr with 20.0 mL of 0.200 M

Ag + .

d. the solution that results from mixing 25.0 mL of 0.0400 M Ag + with 20.0 mL of 0.200 M

KBr.

2. Calculate the theoretical potential of each of the following cells. Is the cell as written

galvanic or electrolytic?

a. Pt ǀ Cr 3+

(2.00 x 10 -4

M), Cr 2+

(1.00 x 10 -3

M) ǁ Pb 2+

(6.50 x 10 -2

M) ǀ Pb

b. Hg ǀ Hg2 2+

(4.00 x 10 -2

M) ǁ H + (3.00 x 10

-2 M), V

3+ (2.00 x 10

-2 M), VO

2+ (6.00 x 10

-3 M) ǀ

Pt

c. Pt ǀ Fe 3+

(2.00 x 10 -2

M), Fe 2+

(6.00 x 10 -5

M) ǁ Sn 2+

(3.50 x 10 -2

M), Sn 4+

(1.50 x 10 -4

M) ǀ Pt

3. The solubility product constant for PbI2 is 7.1 x 10 -9

at 25 o C. Calculate E

o for the process.

4. a. Calculate the standard potential for the reaction

CuBr(s) + e -  Cu(s) + Br

- Ksp = 5.2 x 10

-9

b. Give a schematic representation of a cell with a copper indicator electrode as an anode

and a SCE as a cathode that could be used for the determination of Br - .

c. Derive an equation that relates the measured potential of the cell in (b) to pBr (assume the

junction potential is zero).

d. Calculate the pBr of a bromide-containing solution that is saturated with CuBr and

contained in the cell described in (b) if the resulting potential is Ecell = -0.071 V.

4. The following cell was employed for determination of pCrO4:

Ag ǀ Ag2CrO4(sat’d), CrO4 2-

(x M) ǁ SCE

Calculate pCrO4 if the cell potenital is Ecell = -0.402 V.

5. The standard electrode potential for the reduction of the Cu(II) complex of EDTA is given by

CuY 2-

+ 2 e -  Cu(s) + Y

4- E

o = -0.22 V

Calculate the formation constant for the reaction

Cu 2+

+ Y 4-

 CuY 2-

6. The following cell was found to have a potential of Ecell = 0.124 V:

SCE ǁ Cu 2+

(3.25 x 10 -3

M) ǀ membrane electrode for Cu 2+

When the solution of known copper activity was replaced with an unknown solution, the

potential was found to be Ecell = 0.105 V. What was the pCu of this unknown solution?

Neglect the junction potential.

7. The following cell was found to have a potential of Ecell = 0.2714 V:

SCE ǁ Mg 2+

(a = 3.32 x 10 -3

M) ǀ membrane electrode for Mg 2+

a. When the solution of known magnesium activity was replaced with an unknown solution,

the potential was found to be Ecell = 0.1901 V. what was the pMg of this unknown

solution?

b. Assuming an uncertainity of 0.0020 V in the junction potential, what is the range of

Mg 2+

activities within which the true value might be expected?

c. What is th relative error in [Mg 2+

] associated with the uncertainty in Ej?

8. List the advantages and disadvantages of the dropping mercury electrode compared with

platinum or carbon microelectrodes?

9. Quinone undergoes a reversible reduction at a dropping mercury electrode. The reaction is

C6H4O2 + 2 H + + 2 e

-  C6H6O2 E

o = 0.599 V

a. Assume the diffusion coefficient for quinone and hydroquinone are approximately the

same and calculate the approximate half-wave potential (vs SCE) for the reduction of

hydroquinone at a rotating disk electrode from a solution buffered to a pH of 7.0.

b. Repeat the calculation in (a) for a solution buffered to a pH of 5.0.

10. The following reaction is reversible and has a half-wave potential of -0.349 V when carried

out at a dropping mercury electrode from a solution buffered to pH 2.5.

Ox + 4 H + + 4 e

-  Red

Predicted the half-wave potential at pH:

a. 1.0

b. 3.5

c. 7.0