Show All Work to Receive Credit for Each Problem

CHM 103 NAME ______

GAGE

FALL 2007 December 7, 2007

EXAM 2

SHOW ALL WORK TO RECEIVE CREDIT FOR EACH PROBLEM.

1. The concentration of transition metal ions can be determined by EDTA titration. Below is the reaction of Co3+ with EDTA as the titrant.

Co(NH3)63+ + EDTA4- à Co-EDTA1- + 6 NH3 (aq)

wine-red colorless deep violet colorless

The titration is done spectrophotometrically at the max of Co(NH3)63+. The Co-EDTA-1 complex has a very large formation constant (Kf = 1017).

a. The graph above was generated by titrating 50.0 mL of Co(NH3)63+ with 0.425 M EDTA. What is the initial molar concentration of the cobalt amine complex?

b. On the graph, label the species that cause of the absorbance for the titration at the following points: 1) initially; 2) midway to the equivalence point; 3) at the equivalence point; 4) after the equivalence point.

c. Why would you do this reaction spectrophotometrically rather than visually?

d. Sketch a dashed line (---) on the axes to show what the graph would look like if you followed the reaction at the max for the Co3+-EDTA complex.

2. In the lab you dissolve a 662.3 mg sample of a weak acid in 20.00 mL of water. The resulting solution is titrated with 0.07850 M NaOH and the titration recorded potentiometrically. The resulting graph is displayed below.

a. Locate and label the coordinates of the equivalence point. Record the coordinates here.

b. Determine the molar mass of the weak acid.

c. Calculate the dissociation constant for this acid. Explain how you arrived at this answer from the graph.

3. In the laboratory, you determined the equilibrium constant for the formation of an iron-thiocyanate complex shown by the equation below:

Fe3+ + SCN-  Fe(SCN)2+

By changing the initial iron concentration while keeping the thiocyanate constant, a series of solutions of the complex were produced. The various concentrations of the complex were determined spectrophotometrically using one iron concentration. The equilibrium values of the iron and thiocyanate ions were calculated from the complex and the initial values.

Three students are doing this experiment independently using the same Spec 20 at different times. Student 1 generates an Fe(SCN)2+ standard curve to be able to get the concentrations of the complex. These results are graphed below. Student 2 uses a single standard with a concentration of 4.00 x 10-5 M Fe(SCN)2+ and gets an absorbance of 0.402. Student 3 uses a single standard of 5.00 x 10-5 M Fe(SCN)2+ and gets an absorbance of 0.341.

a. What concentration of Fe(SCN)2+ will each student get for a solution that give an absorbance of 0.520? Be sure to use only a student’s data for his/her answer. Explain how you arrived at each answer.

Student 1 ______Student 2 ______Student 3 ______

b. A 6.00 x 10-5 M solution of Fe(SCN)2+ at the max measured by the instructor on the same instrument used by these students gives an absorbance of 0.660.

Calculate the % error for this solution for students 2 and 3.

c. Which student may have done this procedure at a wavelength slightly higher than the max? Explain your choice.

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