Probset for Experiment 5

General Instruction: Write your answers in a bluebook.

I. Multiple Choice: Write the letter of the best answer (1.5 point each)

1. Which of the following radiation will be mostly absorbed by an orange solution?A. Red visible light B. Yellow visible lightC. Green visible light D. Blue visible light2. This parameter ensures that the spectrophotometric analysis to be sensitive.A. molar extinction coefficient B. path lengthC. analytical wavelength D. transmittance3. To determine the concentration of Fe(SCN)2+ in the unknown solution, a response vs. concentration plot was constructed from the data gathered from a set of standard solutions. This plot is called a/an:A. standard curve B. calibration curveC. normal curve D. best fit line4. Which of the following statements is/are true concerning a blank solution?

I. It contains the same components in the sample solution except the analyteII. It is used to neglect the absorbance of the components other than the analyteIII. It is used to AUTOZERO the spectrophotometer

A. I, II B. I, III C. II, III D. I, II, III5. Which of the following errors is/are systematic?

I. the spectrophotometer was not calibratedII. the measuring pipette used contains an inherent errorIII. the temperature in the spectrophotometer is different to that of the sample preparation room

A. I B. II C. I, II D. I, III

II. Modified TRUE or FALSE: Write TRUE if the statement is true. If it’s not, change the underlined statement/phrase to make the statement correct (2 pts each).

1. An acidic solvent is needed to prepare the 0.002 M FeCl3 solution to suppress the formation of Fe(OH)3 - a possible interference in the absorbance readings.2. Because FeCl3 is added in great excess in the standard solutions, [Fe(SCN)]2+ ≈ [Fe3+]initial.3. A solution containing Fe(SCN)2+ absorbs yellow light.4. The absorbance of the solutions at 466 nm is due to Fe 3+ and Fe(SCN) 2+ 5. Stray light passing through the monochromator is an example of an indeterminate error.

III. Cause and Effect: Determine the effect of the condition on the parameter inside the parenthesis (2 pts each).

1. Distilled water was used as the solvent in preparing the 0.002 M FeCl3 solution, instead of the 0.1 M HCl solution (absorbance of the standard solutions at 466 nm)2. Accidentally, the spectrophotometer is set to 447 nm ([Fe(SCN)2+]eq)3. Instead of 0.2 M, 0.3 M KSCN was used for the preparation of the calibration solutions (calculated Keq).

IV. Problem Solving: Answer the following problem. Show complete solution and box your final answer.

1. The table below is the calibration data for the determination of the complex-formation equilibrium constant of Fe(SCN)2+ using UV-Vis Spectrophotometry. Note that autozero of the blank was done in the measurement and absorbances were gathered at 466 nm.

Test Tube 0.20 M KSCN, mL 0.002 M FeCl3, mL 0.10 M HCl, mL Absorbance1 1.0 0.1 8.9 0.04982 1.0 0.25 8.75 0.14813 1.0 0.5 8.5 0.30124 1.0 1.0 8.0 0.61215 1.0 2.0 7.0 1.2808

a. Write the complex-formation reaction for Fe(SCN)2+

b. Determine the molar extinction coefficient of Fe(SCN)2+ from the calibration datac. From the table below, calculate for the experimental Keq for the formation of Fe(SCN)2+

Test Tube 0.002 M KCSN, mL 0.002 M FeCl3,mL 0.10 M HCl, mL Absorbance1 5.0 3.0 2.0 0.28742 5.0 4.0 1.0 0.3982

2. Spectrophotometric method of analysis was utilized to calculate the equilibrium constant of the formation of tetraamminecopper(II) complex as shown in the reaction below

Cu2+¿+4 N H 3⇌Cu (N H 3)4

2+¿¿ ¿

The following data were taken at λmax = 550 nm and b = 1.00 cmRun Absorbance [Cu2+]initial [NH3]initial

1 0.670 0.001 0.0032 0.740 0.002 0.003

Given that ε550 of Cu(NH3)42+ = 1330 M-1 cm-1,

a. Write the Keq expression for the systemb. Calculate the following in each of the runs:

i. [Cu(NH3)42+]eq

ii. [Cu2+]eq

iii. [NH3]eq

iv. Keq

c. Calculate the average Keq and the % error (lit value of Keq = 1.072 x 1012)