GROUP PORTION
1. Report which wavelength you selected for use in part II.
2. Include a copy of completed Data Tables 1 and 2.
3. Write the equation for the linear fit line from your graph in the form y = mx + b, using the values for m and b that you recorded in the Evidence Record. For example, if m = 3 and b = 6, then the equation for the line is y = 3x + 6.
4. The accuracy of your standard concentrations is indicated by the correlation coefficient (also known as the r value). Report your r value.
5. Show how to use the equation of the line and the measured absorbance of an unknown to
find the concentration of FeSCN2+ as follows:
a. In the equation for the line from question 3, which physical quantity is represented by y and which physical quantity is represented by x?
b. Rewrite the equation so that you have a formula to find concentration from absorbance (isolate concentration on one side of the equation).
c. Choose one of the unknowns you measured and substitute the measured quantities for the unknown into your formula to confirm that the concentration recorded from the interpolation calculator in Logger Pro agrees with the concentration you calculate using your formula.
INDIVIDUAL PORTION
1. Give the identification number of the first unknown solution.
2. Give the unknown concentration as determined from your analysis for the first unknown. If you diluted the unknown, include (and clearly identify) both the diluted and original concentrations.
3. Give the identification number for the second unknown solution.
4. Give the unknown concentration as determined from your analysis. If you diluted the unknown, include (and clearly identify) both the diluted and original concentrations.
For questions 5–7, choose one of the unknown solutions to complete the analysis. Review the obituary document again to understand the story behind the analysis.
5. Based on the calculated concentration of FeSCN2+ in the unknown solution, determine
the concentration of potassium cyanide, KCN, in the original poison solution. Show all your work. Give your answer in milligrams of KCN per milliliter of solution.
Hint: One mole of KCN will produce one mole of FeSCN2+. Assume that all of the KCN in the poisoned solution reacted to form FeSCN2+. Assume that the 20 mL of original solution was not diluted during the reaction to form FeSCN2+ and that the sample you received in lab was also undiluted. The molecular weight of FeSCN2+ is 114 g/mol. The molecular weight of KCN is 65 g/mol.
6. For most people, swallowing 300 mg of KCN is fatal. Based on the concentration of KCN in the poison that you calculated for the unknown you chose (question 2 or 4), determine the approximate volume of cyanide solution that the victim would need to have swallowed for it to have killed him. Show all your work.
7. Assuming about 20 mL of the solution was missing from the vial when found, does it
seem plausible that the 20 mL of solution could have killed the victim had it been slipped
into the coffee?
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