Calculate [FeNCS2+]equil, [Fe3+] initial, [SCN-]initial, [Fe3+]equil, [SCN-]equil, and Kc

Question

1. Standard solution A (~2.0*10^-5)
2. 12.50 mL of 0.200 M Fe(NO₃)₃
3. 2.50 mL of 2.00e-4 M KSCN
4. 10.00 mL of 0.500 M HNO₃
5. Absorbance was 0.02
6. Equilibrium solution 1
7. 10.00 mL of 2.00e-3 M Fe(NO₃)₃
8. 4.00 mL of 2.00e-3 M KSCN
9. 11.00 M of 0.500 M HNO₃
10. Absorbance was 0.09

Please help me to calculate the [FeNCS2+]equil for standard solution A. And also help me calculate [Fe3+] initial, [SCN-]inital, [Fe3+]equil, [SCN-]equil, and the Kc for the equilibrium solution. Any help will be greatly appreciated.

Answer 1

Hello Axel and others,

Thanks for a great question! Let's get to it...

The net ionic equation is Fe3+ + SCN- ---> Fe(NSC)2+. The complex Fe(NSC)2+ is responsible for the color giving the absorbance values. We can tell by inspection for the standard solution that the SNC- ion is the limiting reactant, and since it is added at 2.50 mL out of a total volume of 25.00 mL, its initial concentration in the full solution is 2 x 10^-5 M. We assume that this reaction goes to completion for all practical purposes, since the concentration of Fe3+ is so much higher. Therefore, the concentration of Fe(NCS)2+ at equilibrium in the standard solution is also 2 x 10^-5 M, as noted.

Knowing that the absorbance at a concentation of 0.00 M is zero, we have two points so we can write an linear equation of for the concentration vs. absorbance for Fe(NCS)2+ using Beer's Law. I get

Abs = 1000 [ Fe(NCS)2+] + 0. (Please ask if you have a question about how I got this.)

Now using the new absorbance for the equilibrium solution of 0.09, we find the concentration of Fe(NCS)2+ to be 9 x 10^-5 M by plugging that absorbance into the Beer's Law equation.

We are now in a position to set up an ICE table to solve for the equilibrium concentrations of Fe3+ and SCN-: We use the given concentrations diluted to 25.00 mL to solve for the initial concentrations. (If you would like clarification on this point, please ask. I just use M1* V1 = M2* V2.)

Fe3+ + SCN- ---> Fe(NCS)2+

Initial 0.000800 M 0.000320 M 0

Change - 0.00009 M -0.00009 M + 0.00009 M

Equilibrium 0.00071 M 0.00023 M 0.00009 M

Finally, we can write the Keq expression and solve for the constant.

Keq = [Fe(NSC)2+] / ([Fe3+] [SCN-])

Keq = [0.00009] / ([.00071] [.00023]) = 551.

Since you only reported one significant figure on the absorbance value, our Keq value should probably be rounded to 600.

I hope that helps! I'd love to help you out with any other chemistry problems you might have.