So I am trying to determine the equilibrium constant of the following equation.

Question

So I am trying to determine the equilibrium constant of the following equation.

Fe3+ (aq) + NCS- (aq) ---> FeNCS2+ (aq)

I put following volumes in five test tubes numbered 1-5.

Fe3+ NCS- H2O

1.) 5ml 1ml 4ml

2.) 5ml 2ml 3ml

3.) 5ml 3ml 2ml

4.) 5ml 4ml 1ml

5.) 5ml 5ml 0ml

And then I used Bauch and Lomb spectrometer set at 447 nm and got the following absorbances.

tube 1 0.116

tube 2 0.215

tube 3 0.869

tube 4 0.880

tube 5 1.01

And then I used a "known concentration" to calculate the molarity of FeNCS2+ (which came to be 0.025 M), and used that value to find the molarity of FeNSC2+ in each test tube

(Absorbance known/concentration known) = (Absorbance test tube 1/ concentration test tube 1)

Then I calculate the initial number of moles of Fe3+ and tried to calculate the equilibrium moles of Fe3+ by using (Initial moles of Fe3+) - (Number of moles of FeNCS- at equilibrium),

but I am getting a negative concentration for that. Please check if I have done something wrong to get a negative concentration. Please check if my absorbance values don't add up.

Thanks!

Explanation / Answer

By using the equation given below, which simply says that the ratio of the concentrations is proportional to the ratio of absorbances. We can use c1 to represent the unknown concentration. This equation from Beer's law (Absorbance = e l c)

C1 / C2 = A1 / A2

C1 = (A1 / A2) * C2

for the given absorbance and concentrations

C1 = (0.116 / 0.215) * 0.025

C1 = 1.28 x 10-2 mole cm-3

please try to solve by this way, it will come correct.

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