Question

From her Beer's Law plot in part A, a student obtained a best fit line of y=4.11 x 103x + 0.00 For part B, she made up a series of solutions. One solution had the following composition and absorbance: Vol 2.00 x 10-3 M Fe(NO3)3 (mL)= 5.00 Vol of 2.00 x 10-3 M KSCN (mL)= 5.00 Vol of 0.5 M HNO3 (mL)= 0.00 Absorbance at 450 nm= 0.629 What is the initial concentration of Fe(NO3)3 in the solution (after mixing but before reacting)? M What is the initial concentration of KSCN in the solution (after mixing but before reacting)? M What is the equilibrium concentration of FeSCN2+ determined from the absorbance? M What is the equilibrium concentration of Fe3+ ? M What is the equilibrium concentration of SCN-? M Based on this run, what is the equilibrium constant for the reaction Fe3+(aq) + SCN-(aq) <--> FeSCN2+(aq) ? K=

Answer 1

For the reaction,

Fe3+ + SCN- <==> FeSCN^2+

Part A,

calibration plot best fit line equation,

y = 4.11 x 10^3x + 0

So,

molar absorptivity of FeSCN^2+ = 4110 M-1.cm-1

Part B)

[Fe3+] initial = 0.002 M x 0.005 L/0.010 L = 1 x 10^-3 M

[SCN-] initial = 0.002 M x 0.005 L/0.010 L = 1 x 10^-3 M

Absorbance of solution = 0.629

concentration of FeSCN^2+ = absorbance/molar absorptivity [from Part A]

[FeSCN^2+] equilibrium = 0.629/4110 = 1.53 x 10^-4 M

Equilibrium [Fe3+] = 1 x 10^-3 - 1.53 x 10^-4 = 8.47 x 10^-4 M

Equilibrium [SCN-] = 1 x 10^-3 - 1.53 x 10^-4 = 8.47 x 10^-4 M

Equilibrium constant K,

K = [FeSCN^2+]/[Fe3+][SCN-]

   = 1.53 x 10^-4/(8.47 x 10^-4 x 8.47 x 10^-4)

   = 213.27