Chem 2113 Test 4 Fall 1997
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 solubility product constant for silver bromide is to be determined potentiometrically in the
following cell: SCE||AgBr(saturated), Br-(0.050 M)|Ag
The cell potential is determined to be -0.096 V. Calculate the Ksp for AgBr.
2. A new membrane electrode has been developed for the determination of penicillin by
potentiometric measurement. Solutions of penicillin were prepared, and the potentials for the
solutions were determined vs. the SCE:
Penicillin (M); Potential (mV)
1.0 x 10-6 ; 47.8
1.0 x 10-5 ; 30.6
1.0 x 10-4 ; 14.1
1.0 x 10-3 ; -3.9
Calculate the slope, y-intercept, and correlation coefficient for the linearly-related data.
A 5.00 mL aliquot of a solution containing an unknown amount of penicillin was diluted to 100.0
mL, and the potential of this solution was determined to be 0.0V vs. the SCE. Calculate the
concentration of penicillin in the original solution.
3. The equilibrium constant for the acid/base pair <HIn + H2O In- + H3O+> is 8.00 x 10-5. At
600 nm HIn = 1.23 x 103 and In = 6.96 x 103. Calculate the absorbance at 600 nm of a solution
that initially contains 1.00 x 10-4M HIn. Assume all measurements are in a 1.00 cm cell.
4. The amount of nickel in a sample can be determined by complexing the ion with 2,3-quineoxalinedithiol and measuring the absorbance of the solution at 656 nm. Standard solutions
of nickel were prepared by adding a given volume of 6.50 ppm nickel, 10.00 mL of 2,3-quineoxalinedithiol, and diluting the solutions to 100.0 mL. The %T for these solutions was then
measured, and the results follow:
mL Ni sol'n ; %T
0.00 ; 100.0
5.00 ; 80.0
10.00 ; 64.0
20.00 ; 40.9
50.00 ; 10.7
Calculate the slope and the y-intercept for the linearly-related data.
A 1.500 g sample containing nickel was dissolved in nitric acid and diluted to 500.0 mL. A 1.00
mL aliquot of the unknown was treated with 10.00 mL of 2,3-quineoxalinedithiol and diluted to
100.0 mL. This transmittance for this solution was determined to be 51.7%T. Calculate the
percent nickel in the unknown.
5. At 510 nm, cobalt exhibits a very strong absorption when the ion is complexed with 2,3-quineoxalinedithiol. An 0.0549 g sample containing cobalt was dissolved and diluted to 250 mL.
10.00 mL of this solution was treated with 2,3-quineoxalinedithiol and diluted to 100.00 mL. The
transmittance of this solution was 26.1%T. A second 10.00 mL aliquot was obtained, and 5.00
mL of a 2.00 ppm solution of cobalt was added before treatment with 2,3-quineoxalinedithiol and
dilution to 100.0 mL. This second solution had a transmittance of 19.6%T. Calculate the percent
cobalt in the original sample. All measurements were made in a 1.00 cm cell.
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