Chem 2113 Test 4 Fall 1998

Questions are worth 25 points each. OMIT ONE QUESTION by clearly writing OMIT in the space provided for your work. If you fail to mark OMIT on a question I will omit the last question of the test. Show your work and circle your answers for full credit.

1. The quinhydrone electrode can be used for the potentiometric determination of pH. The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone (Q) and hydroquinone (H2Q), and the potential of the solution is measured with a platinum electrode. The ½ reaction is:

Q + 2H+ + 2e- <==> H2Q E = 0.699V

What is the pH of a solution saturated with quinhydrone if the potential of the platinum electrode in the solution measured against a saturated calomel electrode is 0.175V. Assume the liquid junction potential to be zero.

2. Consider the potentiometric determination of nitrate using a nitrate specific ion electrode. All samples were prepared by a 10x dilution with a constant ionic strength buffer, and the potential vs. the SCE was measured. The results were:

[NO3-] ; E (mV)

1.00 x 10-4 ; -24.7

3.16 x 10-4 ; +4.6

5.00 x 10-4 ; +17.3

1.00 x 10-3 ; +35.0

Calculate the slope, the y-intercept, and the correlation coefficient for the linearly related data.

An unknown containing the nitrate ion was analyzed using the above method. If a 0.543 g sample of the unknown was dissolved and diluted to 100.0 mL with a constant ionic strength buffer and the potential of this solution was found to be -12.4 mV, what is the concentration of nitrate ion in the unknown?

3. Consider the following electrochemical cell:

SCE || AgCl(s), Cl- (0.025 M) | Ag

a) Write the cathodic ½ reaction, and determine the ½ cell potential (vs. the NHE) for the cathode.

b) Write the anodic ½ reaction, and determine the ½ cell potential (vs. the NHE) for the anode.

c) Write the overall reaction, and calculate the overall cell potential.

d) Determine the overall cell potential at equilibrium.

e) What is the equilibrium constant for the reaction in (c)?

4. Consider the titration of 50.0 mL of 0.10 M V3+ with 0.020 M Cr2O72- in an acidic solution.

a) Write the equation for the reaction of Cr2O72- with V3+.

For the following points, calculate the ½ cell potential (vs. the NHE) upon the addition of the specified volume to titrant. Assume [H+] is constant at 1.00 M.

b) After 5.00 mL of titrant is added;

c) After 25.0 mL of titrant is added;

d) After 50.0 mL of titrant is added;

e) After 100.0 mL of titrant is added.

5. Calculate the standard potential for the reaction PbI2(s) + 2e- Pb + 2I- ; given Ksp for PbI2 = 7.9 x 10-9.

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