Question

Absorption Spectrometry: In this lab, you will determine the equilibrium concentration of the colored species ferrothiocyanate (FeSCN2+) in your solutions by spectrometry. The Spectronic 20 machine will pass a beam of 447 nm light through your solution and display its absorbance. The amount of absorption is proportional to the concentration of FeSCN2+. This relationship – true for many solutions – is called "Beer's Law," and has the simple equation: A = bc (eqn. 3) where "A" is the absorption, "b" is 5174.6 for FeSCN2+, and "c" is molarity. Experimental Method: Prepare Burets: At each lab table, there are three empty burets, each labeled and color-coded for the appropriate solution or distilled water. Clean, flush, and fill each with its appropriate solution. You will share these with other groups at your lab table, so be neat and courteous. You may have to add more of a reagent as the burets become empty. Prepare Test Tubes: Clean, dry, and label five large test tubes 1 to 5. Use a test tube rack. Make Five Solutions: Following the mix chart below, add Fe(NO3)3 solution, KSCN solution, and perhaps distilled water into each of the test tubes. The total solution volume in each test tube must be 10.00 mL. Mix with a stirring rod, cleaning between each test tube. Test Tube number 1 2 3 4 5 Fe(NO3)3 sol'n, in mL 5.00 5.00 5.00 5.00 5.00 KSCN sol'n, in mL 1.00 2.00 3.00 4.00 5.00 Distilled H2O, in mL 4.00 3.00 2.00 1.00 - equilibrium V, in mL 10.00 10.00 10.00 10.00 10.00 A student made solution #3 using the experimental method in this lab and measured an absorbance of 0.559. The starting reagents are 2.00 x 10^-3 M Fe(NO3)3 and 2.00 x 10^-3 M KSCN. What is the equilibrium molarity of FeSCN2+ in solution #3? How many moles of FeSCN2+ are in the solution at equilibrium? What are the initial number of moles of FeSCN2+? What are the initial number of moles of each of the reactants? initial moles Fe3+ = initial moles SCN- = Complete the following I.C.E. table (using moles) for the reaction: Fe3+(aq) + SCN-(aq) → FeSCN2+(aq) Fe3+ SCN- FeSCN2+ initial FILL IN FILL IN FILL IN change FILL IN FILL IN FILL IN equilibrium FILL IN FILL IN FILL IN What is the equilibrium molarity of each species in solution #3? [Fe3+] = [SCN-] = [FeSCN2+] = What is the value of Kc for reaction #1?

Absorption Spectrometry: In this lab, you will determine the equilibrium concentration of the colored species ferrothiocyanate (FeSCN2+) in your solutions by spectrometry. The Spectronic 20 machine will pass a beam of 447 nm light through your solution and display its absorbance. The amount of absorption is proportional to the concentration of FeSCN2+. This relationship – true for many solutions – is called "Beer's Law," and has the simple equation: A = bc (eqn. 3) where "A" is the absorption, "b" is 5174.6 for FeSCN2+, and "c" is molarity.

Experimental Method:
Prepare Burets: At each lab table, there are three empty burets, each labeled and color-coded for the appropriate solution or distilled water. Clean, flush, and fill each with its appropriate solution. You will share these with other groups at your lab table, so be neat and courteous. You may have to add more of a reagent as the burets become empty.

Prepare Test Tubes: Clean, dry, and label five large test tubes 1 to 5. Use a test tube rack.

Make Five Solutions: Following the mix chart below, add Fe(NO3)3 solution, KSCN solution, and perhaps distilled water into each of the test tubes. The total solution volume in each test tube must be 10.00 mL. Mix with a stirring rod, cleaning between each test tube.

Test Tube number 1 2 3 4 5
Fe(NO3)3 sol'n, in mL 5.00 5.00 5.00 5.00 5.00
KSCN sol'n, in mL 1.00 2.00 3.00 4.00 5.00
Distilled H2O, in mL 4.00 3.00 2.00 1.00 -
equilibrium V, in mL 10.00 10.00 10.00 10.00 10.00

A student made solution #3 using the experimental method in this lab and measured an absorbance of 0.559. The starting reagents are 2.00 x 10^-3 M Fe(NO3)3 and 2.00 x 10^-3 M KSCN.

What is the equilibrium molarity of FeSCN2+ in solution #3?
How many moles of FeSCN2+ are in the solution at equilibrium?
What are the initial number of moles of FeSCN2+?
What are the initial number of moles of each of the reactants?
initial moles Fe3+ = ____________
initial moles SCN- = ____________

Complete the following I.C.E. table (using moles) for the reaction:
Fe3+(aq) + SCN-(aq) → FeSCN2+(aq)
Fe3+ SCN- FeSCN2+
initial FILL IN FILL IN FILL IN
change FILL IN FILL IN FILL IN
equilibrium FILL IN FILL IN FILL IN

What is the equilibrium molarity of each species in solution #3?
[Fe3+] = ____________
[SCN-] = ____________
[FeSCN2+] = ____________

What is the value of Kc for reaction #1?

Added by Marc S.