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Assignment 4 Page 1

General instructions:

 You must show full/detailed calculations to receive full marks for each problem.

 Please show your FINAL answer to correct number of significant figures. During

intermediate steps, keep all digits in your calculator to avoid large rounding errors in the

final answer. In other words, do not round off your answers during intermediate

calculation steps.

 Write in pen.

 Use KSP, Ka and E o values from textbook – Harris 8

th edition

1. (a) (10 marks) An unknown solution contains only two absorbing species thymol blue and

methylthymol blue. This sample had absorbance of 0.412 at 455 nm and 0.632 at 545 nm when

measured using standard 1.000 cm cell, and the absorbance of blank at the same wavelengths was

0.003 and 0.005 respectively. The table below gives the molar absorptivities of the two molecules:

λ(nm) ε thymol blue (1/Mcm) ε methylthymol blue(1/Mcm)

455 4800 18900

545 36400 4450

Calculate the concentration of thymol blue and methylthymol blue in the unknown sample.

(b) (5 marks) If the unknown solution above contained thymol blue and an unknown compound Y

instead of methylthymol blue, what are the concentrations of the thymol blue and Y given the

molar absorptivities of the two molecules are:

λ(nm) ε thymol blue (1/Mcm) ε Y(1/Mcm)

455 4800 4900

545 36400 36450

Assignment 4 Page 2

2. (5 marks) The titration of a 50.00 mL aqueous sample of Fe 2+

with 0.09533 M Ce 4+

gives the

following cell potentials when using a platinum indicator electrode and silver-silver chloride

reference electrode. Use a Gran plot to determine the concentration of Fe 2+

in this sample.

Volume of added Titrant (mL)

E (V)

32.00 0.600

34.00 0.608

36.00 0.617

38.00 0.629

40.00 0.649

42.00 1.355

46.00 1.444

50.00 1.461

3. The following results were obtained for a spectrophotometric titration that was carried out at 600

nm to measure Cu 2+

in a water sample according to the reaction below:

Cu 2+

+ Trien → Cu(trien) 2+

A 10.00 mL sample of water was used for this titration, and the following results were obtained upon the

addition of increments of titrant to the sample:

Sample volume (mL)

Volume Trien (mL) Absorbance

0.00 0.00 0.002

10.00 0.00 0.005

10.00 2.00 0.181

10.00 4.00 0.306

10.00 6.00 0.402

10.00 8.00 0.416

10.00 10.00 0.375

(a) (10 marks) What was the concentration of Cu 2+

in the original sample?

(b) (2 marks) Explain the shape of the titration curve obtained for this analysis. What does this curve

tell you about the ability of the analyte, titrant and product to absorb light at the wavelength used

for this titration?

(c) (3 marks) Use the information provided to estimate the value of the molar absorptivities for the

analyte, titrant and product at 600 nm.

Assignment 4 Page 3

4. (15 marks) Given the following spectra of ibuprofen and sulindac, in numbered bullet format

explain how you would perform the analysis of a tablet sample containing unknown amounts of

ibuprofen and sulindac. For full marks, you have to be very specific and describe all steps in

detail.

UV spectra are copyright Photostability of drugs and drug formulations, 2 nd

edition, by H.H. Tonessen

(p.11)

Assignment 4 Page 4

5. (a) (9 marks) Calibration data for a F - ion-selective electrode using KF standard solutions prepared in

buffer of 0.5 M ionic strength is given in the table below. This fluoride ion-selective electrode has

selectivity coefficient of 0.001 for chloride ions. Construct calibration curve, perform least-squares linear

regression as appropriate and determine the magnitude of β for this particular electrode.

[KF] (M) E (mV)

1.00 0.0

1.00 x 10 -1

52.2

1.00 x 10 -2

106.1

1.00 x 10 -3

161.7

1.00 x 10 -4

217.4

1.00 x 10 -5

276.1

1.00 x 10 -6

323.0

1.00 x 10 -7

340.0

1.00 x 10 -8

345.0

(b) (3 marks) An unknown sample of tap water was analyzed 5 hours after the above calibration data was

collected. E measured was 341.0 mV. What is the concentration of F - in this tap water sample? How

accurate is this concentration and explain your reasoning?

(c) (3 marks) An unknown sample of ocean water was analyzed immediately after the above calibration

data was collected. E measured was 142.0 mV. What is the concentration of F - in this ocean water

sample? How accurate is this concentration and explain your reasoning?

(d) (5 marks) What (if anything) would you do differently if you were performing the analyses described in

(b) and (c) to increase the accuracy of the [F - ] determination? For full marks, be very clear and specific

6. (10 marks) Compound P binds with X to form the complex PX. A series of solutions was prepared with

the total P concentration remaining fixed at 1.00 x10 -5

M in all solutions. Both P and X do not absorb at

437 nm, but PX has an absorption maximum at 437 nm. The following table shows how the absorbance

at 437 nm in a 5.00 cm cell depends of [X], using the assumption that [X]is approximately equal to XT

because XT >>*P+. Make a Scatchard plot of ΔA/*X+ versus ΔA in order to find the equilibrium constant K

for this binding event.

[X] in M A

No X added 0.000

0.00200 0.125

0.00400 0.213

0.00600 0.286

0.00800 0.342

0.0100 0.406

0.0200 0.535

0.0400 0.631

0.0600 0.700

0.0800 0.708

0.1000 0.765