For the equilibrium solution in 5 above, the concentration of the Fe(SCN)2 produ
ID: 560527 • Letter: F
Question
For the equilibrium solution in 5 above, the concentration of the Fe(SCN)2 product is determined by measuring the absorption of the solution, and calculating the concentration of Fe(SCN)2. responsible for that absorption from the calibration equation. Why must [Fe(SCN)2+] in this solution be determined from a measured absorption and the calibration equation, rather than from the initial concentration of F33+, as is done for the solution in question 4 above? 6b) A student measured the absorption of the solution in 5 above is measured, and found it to be 0.135 AU's (absorption units). If the student's calibration equation relating concentration and absorption on their spectrometer was found to be Absorptions 4585 [Fe(SCN)2+1+ 3. where the units of the slope are AU/M and the units of the intercept are AU What would be the concentration of Fe(SCN)2 in this solution? 7) Given the initial concentrations of Fe and SCN found for the solution in 5 above, and the equilibrium concentration of Fe(SCN)2. found in 6b, use thelCE table below to determine the equilibrium concentration of Fe3 and of SCN, and calculate a value for the equilibrium constant for this reaction. Initial molarity Change in molarity Equilibrium molarity Solutio Schulin,Hal = 3.5Explanation / Answer
Concentration of Fe(SCN)2+
6b. For a solution of Fe3+ + SCN- ---> Fe(SCN)2+
the absorbance value was found = 0.135 AU
The calibration curve equation was found to be,
Absorbance = 4585[Fe(SCN)2+] + 3
slope = molar absorptivity = 4585 M-1.cm-1
path length = 1 cm
absorbance = molar absorptivity x path length x concentration
or,
[Fe(SCN)2+ concentration = 0.135/4585 = 2.94 x 10^-5 M
7) For the complexation reaction,
initial [Fe3+] = 3.571 x 10^-4 M
initial [SCN-] = 3.571 x 10^-4 M
ICE chart
Fe3+ + SCN- <===========> Fe(SCN)2+
initial molarity 3.571 x 10^-4 3.571 x 10^-4 -
change in molarity 2.94 x 10^-5 2.94 x 10^-5 +2.94 x 10^-5
equilibrium molarity 3.3 x 19^-4 3.3 x 10^-4 2.94 x 10^-5
So,
equilibrium constant (Kc) = [Fe(SCN)2+/[Fe3+][SCN-]2+
= (2.94 x 10^-5)/(3.3 x 10^-4)^2
= 272
So equilibrium constant for the reaction is 272